Adds onPlaybackRateChange info to README

[JoeM-RP]

Describe the changes

Added a brief summary of 'onPlaybackRateChange' to the README based on comments for #193

[CHaNGeTe]

Thanks for adding it to the README
AFAIK playbackRate can also be, for example, 0.5

[JoeM-RP]

@CHaNGeTe - any sense of what state might be indicated by 0.5? I can update the language to be more explicit

[CHaNGeTe]

Isn't it related to rate prop?

https://github.com/react-native-community/react-native-video#rate

https://github.com/react-native-community/react-native-video/blob/master/android-exoplayer/src/main/java/com/brentvatne/exoplayer/ReactExoplayerView.java#L767

https://github.com/react-native-community/react-native-video/blob/master/ios/Video/RCTVideo.m#L700

In that case, 0.5 is reproduce the stream at half speed, for example.