Add release notes

Author: tonihei

RELEASENOTES.md

@@ -117,8 +117,8 @@
         ([#710](https://github.com/androidx/media/pull/710)).
     *   MIDI: Fix issue where seeking forward skips the Program Change events
         ([#704](https://github.com/androidx/media/issues/704).
-    *   Migrate to FFmpeg 6.0
-        ([#707](https://github.com/androidx/media/pull/707)).
+    *   Migrate to FFmpeg 6.0 and update supported NDK to `r26b`
+        ([#707](https://github.com/androidx/media/pull/707), [#867](https://github.com/androidx/media/pull/867)).
 *   Leanback extension:
 *   Cast Extension:
     *   Sanitize creation of a `Timeline` to not crash the app when loading