Add license notes for extensions with non-Google dependencies

Issue: #3197

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=166988657

extensions/ffmpeg/README.md

@@ -3,6 +3,14 @@
 The FFmpeg extension provides `FfmpegAudioRenderer`, which uses FFmpeg for
 decoding and can render audio encoded in a variety of formats.
 
+## License note ##
+
+Please note that whilst the code in this repository is licensed under
+[Apache 2.0][], using this extension also requires building and including one or
+more external libraries as described below. These are licensed separately.
+
+[Apache 2.0]: https://github.com/google/ExoPlayer/blob/release-v2/LICENSE
+
 ## Build instructions ##
 
 To use this extension you need to clone the ExoPlayer repository and depend on

extensions/flac/README.md

@@ -3,6 +3,14 @@
 The Flac extension provides `FlacExtractor` and `LibflacAudioRenderer`, which
 use libFLAC (the Flac decoding library) to extract and decode FLAC audio.
 
+## License note ##
+
+Please note that whilst the code in this repository is licensed under
+[Apache 2.0][], using this extension also requires building and including one or
+more external libraries as described below. These are licensed separately.
+
+[Apache 2.0]: https://github.com/google/ExoPlayer/blob/release-v2/LICENSE
+
 ## Build instructions ##
 
 To use this extension you need to clone the ExoPlayer repository and depend on

extensions/okhttp/README.md

@@ -6,6 +6,14 @@ The OkHttp extension is an [HttpDataSource][] implementation using Square's
 [HttpDataSource]: https://google.github.io/ExoPlayer/doc/reference/com/google/android/exoplayer2/upstream/HttpDataSource.html
 [OkHttp]: https://square.github.io/okhttp/
 
+## License note ##
+
+Please note that whilst the code in this repository is licensed under
+[Apache 2.0][], using this extension requires depending on OkHttp, which is
+licensed separately.
+
+[Apache 2.0]: https://github.com/google/ExoPlayer/blob/release-v2/LICENSE
+
 ## Getting the extension ##
 
 The easiest way to use the extension is to add it as a gradle dependency:

extensions/opus/README.md

@@ -3,6 +3,14 @@
 The Opus extension provides `LibopusAudioRenderer`, which uses libopus (the Opus
 decoding library) to decode Opus audio.
 
+## License note ##
+
+Please note that whilst the code in this repository is licensed under
+[Apache 2.0][], using this extension also requires building and including one or
+more external libraries as described below. These are licensed separately.
+
+[Apache 2.0]: https://github.com/google/ExoPlayer/blob/release-v2/LICENSE
+
 ## Build instructions ##
 
 To use this extension you need to clone the ExoPlayer repository and depend on

extensions/rtmp/README.md

@@ -7,6 +7,14 @@ 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
 
+## License note ##
+
+Please note that whilst the code in this repository is licensed under
+[Apache 2.0][], using this extension requires depending on LibRtmp Client for
+Android, which is licensed separately.
+
+[Apache 2.0]: https://github.com/google/ExoPlayer/blob/release-v2/LICENSE
+
 ## Getting the extension ##
 
 The easiest way to use the extension is to add it as a gradle dependency:

extensions/vp9/README.md

@@ -3,6 +3,14 @@
 The VP9 extension provides `LibvpxVideoRenderer`, which uses libvpx (the VPx
 decoding library) to decode VP9 video.
 
+## License note ##
+
+Please note that whilst the code in this repository is licensed under
+[Apache 2.0][], using this extension also requires building and including one or
+more external libraries as described below. These are licensed separately.
+
+[Apache 2.0]: https://github.com/google/ExoPlayer/blob/release-v2/LICENSE
+
 ## Build instructions ##
 
 To use this extension you need to clone the ExoPlayer repository and depend on