Update SDK README to discuss about:tracing and observatory.

sky/sdk/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.
 
 Sky consists of two components:
 
 1. *The Sky engine.* The engine is the core of the system. Written in C++, the
@@ skipping 115 matching lines @@
 
 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.
 
-1. ``packages/sky/lib/sky_tool start --install examples/stocks/main.sky``
+1. ``packages/sky/sky_tool start --install examples/stocks/main.sky``
    The --install flag is only necessary to install SkyDemo.apk if not already
    installed from the Google Play store.
 
 2.  Use ``adb logcat`` to view any errors or Dart print() output from the app.
 
+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 [a bug](https://github.com/domokit/mojo/issues/127) this 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
@@ skipping 15 matching lines @@
 [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.