Update the native-activity README.

I changed what this sample does but forgot to update the docs.

Author: DanAlbert

native-activity/README.md

@@ -1,8 +1,24 @@
 # Native Activity
 
-Native Activity is an Android sample that initializes a GLES 2.0 context and
-reads accelerometer data from C code using
-[Native Activity](http://developer.android.com/reference/android/app/NativeActivity.html).
+This is an Android sample that uses [NativeActivity] with `native_app_glue`,
+which enables building NDK apps without having to write any Java code. In
+practice most apps, even games which are predominantly native code, will need to
+call some Java APIs or customize their app's activity further.
+
+The more modern approach to this is to use [GameActivity], which has all the
+same benefits as `NativeActivity` and `native_app_glue`, while also making it
+easier to include Java code in your app without a rewrite later. It's also
+source compatible. This sample will likely migrate to `GameActivity` in the
+future.
+
+The app here is intentionally quite simple, aiming to show the core event and
+draw loop necessary for an app using `native_app_glue` without any extra
+clutter. It uses `AChoreographer` to manage the update/render loop, and uses
+`ANativeWindow` and `AHardwareBuffer` to update the screen with a simple color
+clear.
+
+[GameActivity]: https://developer.android.com/games/agdk/game-activity
+[NativeActivity]: http://developer.android.com/reference/android/app/NativeActivity.html
 
 This sample uses the new
 [Android Studio CMake plugin](http://tools.android.com/tech-docs/external-c-builds)