Updated RELEASE_NOTES.

RELEASE_NOTES.md

@@ -8,6 +8,8 @@ Added Dutch (nl_NL) translation (contributed by martijndeb).
 
 Added Western Frisian (fy_NL) translation (contributed by martijndeb).
 
+Mastodon API: Fixed repeated entries in timelines.
+
 ## 2.81
 
 If the `propagate_local_purge` configuration variable is set to `true` in `server.json`, purged local posts generate a `Delete` activity that is sent everywhere, instead of only deleted from the filesystem.