add the sky sdk README back in

sky/sdk/README.md

+Contributing
+============
+
+[sky_sdk](https://github.com/domokit/sky_sdk) is generated from the
+[mojo repository](https://github.com/domokit/mojo) using
+[deploy_sdk.py](https://github.com/domokit/mojo/blob/master/sky/tools/deploy_sdk.py)
+Static files (including this README.md) are located under
+[sky/sdk](https://github.com/domokit/mojo/tree/master/sky/sdk).  Pull
+requests and issue reports are glady accepted at the
+[mojo repository](https://github.com/domokit/mojo)!
+
+Sky
+===
+
+Sky is an experimental, high-performance UI framework for mobile apps. Sky helps
+you create apps with beautiful user interfaces and high-quality interactive
+design that run smoothly at 120 Hz.
+
+Sky consists of two components:
+
+1. *The Sky engine.* The engine is the core of the system. Written in C++, the
+   engine provides the muscle of the Sky system. The engine provides
+   several primitives, including a soft real-time scheduler and a hierarchical,
+   retained-mode graphics system, that let you build high-quality apps.
+
+2. *The Sky framework.* The [framework](packages/sky/lib/framework) makes it
+   easy to build apps using Sky by providing familiar user interface widgets,
+   such as buttons, infinite lists, and animations, on top of the engine using
+   Dart. These extensible components follow a functional programming style
+   inspired by [React](http://facebook.github.io/react/).
+
+We're still iterating on Sky heavily, which means the framework and underlying
+engine are both likely to change in incompatible ways several times, but if
+you're interested in trying out the system, this document can help you get
+started.
+
+Examples
+--------
+
+Sky uses Dart and Sky applications are
+[Dart Packages](https://www.dartlang.org/docs/tutorials/shared-pkgs/).
+Application creation starts by creating a new directory and
+adding a [pubspec.yaml](https://www.dartlang.org/tools/pub/pubspec.html):
+
+ pubspec.yaml for your app:
+```yaml
+name: your_app_name
+dependencies:
+ sky: any
+```
+
+Once the pubspec is in place, create a `lib` directory (where your dart code
+will go), ensure that the 'dart' and 'pub' executables are on your $PATH and
+run the following:
+
+`pub get && pub run sky:init`.
+
+Currently the Sky Engine assumes the entry point for your application is a
+`main` function located inside a `main.sky` file at the root of the package.
+`.sky` is an html-like format:
+```
+<sky>
+<script>
+import 'package:your_app_name/main.dart';
+
+void main() {
+  new HelloWorldApp();
+}
+</script>
+</sky>
+```
+
+The rest of the application then goes inside the `lib` directory of the package
+thus `lib/main.dart` would be:
+
+```dart
+import 'package:sky/framework/fn.dart';
+
+class HelloWorldApp extends App {
+  UINode build() {
+    return new Text('Hello, world!');
+  }
+}
+```
+
+Execution starts in `main`, which creates the `HelloWorldApp`. The framework
+then marks `HelloWorldApp` as dirty, which schedules it to build during the next
+animation frame. Each animation frame, the framework calls `build` on all the
+dirty components and diffs the virtual `UINode` hierarchy returned this frame
+with the hierarchy returned last frame. Any differences are then applied as
+mutations to the physical hierarchy retained by the engine.
+
+Skip down to "Running a Sky application" to learn how to load and run this
+example on your device.
+
+For examples, please see the [examples directory](examples/).
+
+Services
+--------
+
+Sky apps can access services from the host operating system using Mojo IPC. For
+example, you can access the network using the `network_service.mojom` interface.
+Although you can use these low-level interfaces directly, you might prefer to
+access these services via libraries in the framework. For example, the
+`fetch.dart` library wraps the underlying `network_service.mojom` in an
+ergonomic interface:
+
+```dart
+import 'package:sky/framework/net/fetch.dart';
+
+main() async {
+  Response response = await fetch('example.txt');
+  print(response.bodyAsString());
+}
+```
+
+Set up your computer
+--------------------
+
+1. Install the Dart SDK:
+  - https://www.dartlang.org/tools/download.html
+
+2. Install the ``adb`` tool from the Android SDK:
+  - https://developer.android.com/sdk/installing/index.html
+
+3. Install the Sky SDK:
+  - ``git clone https://github.com/domokit/sky_sdk.git``
+
+4. Ensure that $DART_SDK is set to the path of your Dart SDK and 'adb'
+   (inside 'platform-tools' in the android sdk) is in your $PATH.
+
+Set up your device
+------------------
+
+Currently Sky requires an Android device running the Lollipop (or newer) version
+of the Android operating system.
+
+1. Enable developer mode on your device by visiting ``Settings > About phone``
+   and tapping the ``Build number`` field five times.
+
+2. Enable ``USB debugging`` in ``Settings > Developer options``.
+
+3. Using a USB cable, plug your phone into your computer. If prompted on your
+   device, authorize your computer to access your device.
+
+Running a Sky application
+-------------------------
+
+The `sky` pub package includes a `sky_tool` script to assist in running
+Sky applications inside the `SkyDemo.apk` harness.  The sky_tool script expects
+to be run from the root directory of your application pub package.  To run
+one of the examples in this SDK, try:
+
+1. ``cd examples/stocks``
+
+2. ``pub get`` to set up a copy of the sky package in the app directory.
+
+3. ``./packages/sky/sky_tool start`` to start the dev server and upload your
+   app to the device.
+   (NOTE: add a ``--install`` flag to install SkyDemo.apk if not already
+   installed on the device.)
+
+4. Use ``adb logcat`` to view any errors or Dart print() output from the app.
+   ``adb logcat -s chromium`` can be used to filter only adb messages from
+   `SkyDemo.apk` (which for
+   [legacy reasons](https://github.com/domokit/mojo/issues/129) still uses the
+   android log tag 'chromium').
+
+Measuring Performance
+---------------------
+
+Sky has support for generating trace files compatible with
+[Chrome's about:tracing](https://www.chromium.org/developers/how-tos/trace-event-profiling-tool).
+
+`packages/sky/sky_tool start_tracing` and `packages/sky/sky_tool stop_tracing`
+are the commands to use.
+
+Due to https://github.com/domokit/mojo/issues/127 tracing currently
+requires root access on the device.
+
+Debugging
+---------
+
+Dart's [Observatory](https://www.dartlang.org/tools/observatory/)
+(VM Debugger & Profiler) support in Sky is
+[in progress](https://codereview.chromium.org/1107803002) and should
+be released shortly after Dart Summit 2015.
+
+Building a standalone MyApp
+---------------------------
+
+Although it is possible to bundle the Sky Engine in your own app (instead of
+running your code inside SkyDemo.apk), right now doing so is difficult.
+
+There is one example of doing so if you're feeling brave:
+https://github.com/domokit/mojo/tree/master/sky/apk/stocks
+
+Eventually we plan to make this much easier and support platforms other than
+Android, but that work is yet in progress.
+
+Adding Services to MyApp
+------------------------
+
+[Mojo IPC](https://github.com/domokit/mojo) is an inter-process-communication
+system designed to provide cross-thread, cross-process, and language-agnostic
+communication between applications.  Sky uses Mojo IPC to make it possible
+to write UI code in Dart and yet depend on networking code, etc. written in
+another language.  Services are replicable, meaning that Dart code
+written to use the `network_service` remains portable to any platform
+(iOS, Android, etc.) by simply providing a 'natively' written `network_service`.
+
+Embedders of the Sky Engine and consumers of the Sky Framework can use this
+same mechanism to expose not only existing services like the
+[Keyboard](https://github.com/domokit/mojo/blob/master/mojo/services/keyboard/public/interfaces/keyboard.mojom)
+service to allow Sky Framework Dart code to interface with the underlying
+platform's Keyboard, but also to expose any additional non-Dart business logic
+to Sky/Dart UI code.
+
+As and example, [SkyApplication](https://github.com/domokit/mojo/blob/master/sky/shell/org/domokit/sky/shell/SkyApplication.java)
+exposes a mojo `network_service` (required by Sky Engine C++ code)
+[SkyDemoApplication](https://github.com/domokit/mojo/blob/master/sky/apk/demo/org/domokit/sky/demo/SkyDemoApplication.java)
+additionally exposes `keyboard_service` and `sensor_service` for use by the Sky
+Framework from Dart.