Add CHANGELOG entry

CHANGELOG

@@ -40,6 +40,7 @@ v 8.12.0 (unreleased)
   - Use the default branch for displaying the project icon instead of master !5792 (Hannes Rosenögger)
   - Adds response mime type to transaction metric action when it's not HTML
   - Fix hover leading space bug in pipeline graph !5980
+  - Instructions for enabling Git packfile bitmaps !6104
 
 v 8.11.3 (unreleased)
   - Allow system info page to handle case where info is unavailable