Reinstate gradle instructions for IMA extension

Note: Wont work quite yet; we need to actually do a
release for the instructions to become effective.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162220939

extensions/ima/README.md

@@ -11,8 +11,17 @@ alongside content.
 
 ## Getting the extension ##
 
-To use this extension you need to clone the ExoPlayer repository and depend on
-its modules locally. Instructions for doing this can be found in ExoPlayer's
+The easiest way to use the extension is to add it as a gradle dependency:
+
+```gradle
+compile 'com.google.android.exoplayer:extension-ima:rX.X.X'
+```
+
+where `rX.X.X` is the version, which must match the version of the ExoPlayer
+library being used.
+
+Alternatively, you can clone the ExoPlayer repository and depend on the module
+locally. Instructions for doing this can be found in ExoPlayer's
 [top level README][].
 
 [top level README]: https://github.com/google/ExoPlayer/blob/release-v2/README.md