Update IMA extension README
Update IMA extension README

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162742982

saki4510t/ExoPlayer,amzn/exoplayer-amazon-port,saki4510t/ExoPlayer,amzn/exoplayer-amazon-port,google/ExoPlayer,tntcrowd/ExoPlayer,MaTriXy/ExoPlayer,ened/ExoPlayer,ebr11/ExoPlayer,kiall/ExoPlayer,MaTriXy/ExoPlayer,androidx/media,kiall/ExoPlayer,KiminRyu/ExoPlayer,stari4ek/ExoPlayer,ened/ExoPlayer,superbderrick/ExoPlayer,ebr11/ExoPlayer,superbderrick/ExoPlayer,tntcrowd/ExoPlayer,tntcrowd/ExoPlayer,androidx/media,KiminRyu/ExoPlayer,google/ExoPlayer,MaTriXy/ExoPlayer,saki4510t/ExoPlayer,stari4ek/ExoPlayer,amzn/exoplayer-amazon-port,ebr11/ExoPlayer,androidx/media,ened/ExoPlayer,stari4ek/ExoPlayer,KiminRyu/ExoPlayer,google/ExoPlayer,superbderrick/ExoPlayer,kiall/ExoPlayer