Update Readme to include onBuffer event (#2141)

README.md

@@ -340,6 +340,7 @@ var styles = StyleSheet.create({
 |--|--|
 |[onAudioBecomingNoisy](#onaudiobecomingnoisy)|Android ExoPlayer, iOS|
 |[onBandwidthUpdate](#onbandwidthupdate)|Android ExoPlayer|
+|[onBuffer](#onbuffer)|Android ExoPlayer, iOS|
 |[onEnd](#onend)|All|
 |[onExternalPlaybackChange](#onexternalplaybackchange)|iOS|
 |[onFullscreenPlayerWillPresent](#onfullscreenplayerwillpresent)|Android ExoPlayer, Android MediaPlayer, iOS|
@@ -955,6 +956,24 @@ Note: On Android ExoPlayer, you must set the [reportBandwidth](#reportbandwidth)
 
 Platforms: Android ExoPlayer
 
+#### onBuffer
+Callback function that is called when the player buffers.
+
+Payload:
+
+Property | Type | Description
+--- | --- | ---
+isBuffering | boolean | Boolean indicating whether buffering is active
+
+Example:
+```
+{
+  isBuffering: true
+}
+```
+
+Platforms: Android ExoPlayer, iOS
+
 #### onEnd
 Callback function that is called when the player reaches the end of the media.