Add release notes for github.com//pull/2259

Author: ivanbuper

RELEASENOTES.md

@@ -47,6 +47,8 @@
     *   Fix bug that `AnalyticsListener.onAudioPositionAdvancing` was not
         reporting the time when the audio started advancing but the time of the
         first measurement.
+    *   Add support for all linear PCM sample formats in
+        `ChannelMappingAudioProcessor` and `TrimmingAudioProcessor`.
 *   Video:
     *   Add experimental `ExoPlayer` API to include the
         `MediaCodec.BUFFER_FLAG_DECODE_ONLY` flag when queuing decode-only input