Add gradle instructions to GVR readme + clean up FFMPEG readme

Note: Depending on the GVR extension via gradle wont work until
we actually push a release

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=150208524

extensions/ffmpeg/README.md

@@ -31,9 +31,7 @@ FFMPEG_EXT_PATH="${EXOPLAYER_ROOT}/extensions/ffmpeg/src/main"
 NDK_PATH="<path to Android NDK>"
 ```
 
-* Fetch and build FFmpeg.
-
-For example, to fetch and build for armv7a:
+* Fetch and build FFmpeg. For example, to fetch and build for armv7a:
 
 ```
 cd "${FFMPEG_EXT_PATH}/jni" && \
@@ -69,15 +67,14 @@ make -j4 && \
 make install-libs
 ```
 
-* Build the JNI native libraries.
+* Build the JNI native libraries. Repeat this step for any other architectures
+  you need to support.
 
 ```
 cd "${FFMPEG_EXT_PATH}"/jni && \
 ${NDK_PATH}/ndk-build APP_ABI=armeabi-v7a -j4
 ```
 
-Repeat these steps for any other architectures you need to support.
-
 * In your project, you can add a dependency on the extension by using a rule
   like this:
 

extensions/gvr/README.md

@@ -6,13 +6,33 @@ The GVR extension wraps the [Google VR SDK for Android][]. It provides a
 GvrAudioProcessor, which uses [GvrAudioSurround][] to provide binaural rendering
 of surround sound and ambisonic soundfields.
 
-## Instructions ##
+## Using the extension ##
 
-If using SimpleExoPlayer, override SimpleExoPlayer.buildAudioProcessors to
-return a GvrAudioProcessor.
+The easiest way to use the extension is to add it as a gradle dependency. You
+need to make sure you have the jcenter repository included in the `build.gradle`
+file in the root of your project:
 
-If constructing renderers directly, pass a GvrAudioProcessor to
-MediaCodecAudioRenderer's constructor.
+```gradle
+repositories {
+    jcenter()
+}
+```
+
+Next, include the following in your module's `build.gradle` file:
+
+```gradle
+compile 'com.google.android.exoplayer:extension-gvr:rX.X.X'
+```
+
+where `rX.X.X` is the version, which must match the version of the ExoPlayer
+library being used.
+
+## Using GvrAudioProcessor ##
+
+* If using SimpleExoPlayer, override SimpleExoPlayer.buildAudioProcessors to
+  return a GvrAudioProcessor.
+* If constructing renderers directly, pass a GvrAudioProcessor to
+  MediaCodecAudioRenderer's constructor.
 
 [Google VR SDK for Android]: https://developers.google.com/vr/android/
 [GvrAudioSurround]: https://developers.google.com/vr/android/reference/com/google/vr/sdk/audio/GvrAudioSurround