Clarify contribution branch
Clarify contribution branch

PiperOrigin-RevId: 222632883

google/ExoPlayer,saki4510t/ExoPlayer,androidx/media,androidx/media,stari4ek/ExoPlayer,stari4ek/ExoPlayer,superbderrick/ExoPlayer,ened/ExoPlayer,amzn/exoplayer-amazon-port,superbderrick/ExoPlayer,ened/ExoPlayer,superbderrick/ExoPlayer,androidx/media,amzn/exoplayer-amazon-port,amzn/exoplayer-amazon-port,google/ExoPlayer,stari4ek/ExoPlayer,saki4510t/ExoPlayer,saki4510t/ExoPlayer,ened/ExoPlayer,google/ExoPlayer