Update changelog.

Author: cmard

com.unity.ml-agents/CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to
 ### Minor Changes
 #### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
 - Added the capacity to initialize behaviors from any checkpoint and not just the latest one (#5525)
+- Agent can now view the StackedVectorObservations (#5523)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - Set gym version in gym-unity to gym release 0.20.0
 ### Bug Fixes