Add a blurb to the SDK README about Mojo IPC and Services.

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 132 matching lines @@
 ---------------------------
 
 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 replacable, 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.