Add CHANGELOG entry [ci skip]

CHANGELOG

@@ -37,6 +37,7 @@ v 8.10.0 (unreleased)
   - Fix viewing notification settings when a project is pending deletion
   - Updated compare dropdown menus to use GL dropdown
   - Eager load award emoji on notes
+  - Allow to define manual actions/builds on Pipelines and Environments
   - Fix pagination when sorting by columns with lots of ties (like priority)
   - The Markdown reference parsers now re-use query results to prevent running the same queries multiple times !5020
   - Updated project header design