Document usage of the RTMP extension

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=164706135

extensions/rtmp/README.md

@@ -9,7 +9,7 @@ streams using [LibRtmp Client for Android][].
 [RTMP]: https://en.wikipedia.org/wiki/Real-Time_Messaging_Protocol
 [LibRtmp Client for Android]: https://github.com/ant-media/LibRtmp-Client-for-Android
 
-## Using the extension ##
+## Getting the extension ##
 
 The easiest way to use the extension is to add it as a gradle dependency:
 
@@ -25,3 +25,19 @@ 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
+
+## Using the extension ##
+
+ExoPlayer requests data through `DataSource` instances. These instances are
+either instantiated and injected from application code, or obtained from
+instances of `DataSource.Factory` that are instantiated and injected from
+application code.
+
+`DefaultDataSource` will automatically use uses the RTMP extension whenever it's
+available. Hence if your application is using `DefaultDataSource` or
+`DefaultDataSourceFactory`, adding support for RTMP streams is as simple as
+adding a dependency to the RTMP extension as described above. No changes to your
+application code are required. Alternatively, if you know that your application
+doesn't need to handle any other protocols, you can update any `DataSource`s and
+`DataSource.Factory` instantiations in your application code to use
+`RtmpDataSource` and `RtmpDataSourceFactory` directly.