Improve Sky READMEs

sky/README.md

 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.
+For information about using Sky, please see the
+[Sky SDK](https://github.com/domokit/sky_sdk/).
 
-Sky consists of two components:
-
-1. *The Sky engine.* The [engine](engine) is the core of the system 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
-   hierarchial, retained-mode graphics system, that let you build high-quality
-   apps.
-
-2. *The Sky framework.* The [framework](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.
-
-Sky is still experimental. We're experimenting with different ideas and
-exploring various approaches, many of which won't work and will need to be
-discarded, but, if we're lucky, some of which might turn out to be useful.
-
-Examples
---------
-
-The simplest Sky app is, appropriately, HelloWorldApp:
-
-```dart
-import 'package:sky/framework/fn.dart';
-
-class HelloWorldApp extends App {
-  Node build() {
-    return new Text('Hello, world!');
-  }
-}
-
-void main() {
-  new HelloWorldApp();
-}
-```
-
-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 `Node` hierarchy returned this frame with
-the hierarchy returned last frame. Any differences are then applied as mutations
-to the physical heiarchy retained by the engine.
-
-For a more featureful example, please see the
-[example stocks app](examples/stocks/lib/stocks_app.dart).
-
-Services
---------
-
-Sky apps can access services from the host operating system using Mojo. 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';
-
-void foo() {
-  fetch('example.txt').then((Response response) {
-    print(response.bodyAsString());
-  });
-}
-```
-
-Supported platforms
--------------------
-
-Currently, Sky supports the Android and Mojo operating systems.
-
-Specifications
---------------
-
-We're documenting Sky with a [set of technical specifications](specs) that
-define precisely the behavior of the engine.  Currently both the implementation
-and the specification are in flux, but hopefully they'll converge over time.
-
-Contributing
-------------
-
-Instructions for building and testing Sky are contained in [HACKING.md](HACKING.md).
+For information about contributing to Sky, please see [HACKING.md](HACKING.md).