Add a note at the top of the SDK README.md to explain how to contribute to the Sky SDK.

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
@@ skipping 186 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.