Add changelog for !3519

CHANGELOG.md

@@ -17,6 +17,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Unreleased-patch
 - Mastodon API: Activity Search fallbacks on status fetching after a DB Timeout/Error
+- Make activity search properly use GIN indexes
 
 ## 2.4.0 - 2021-08-xx