[docs] Update changelog (#5095)

com.unity.ml-agents/CHANGELOG.md

 and this project adheres to
 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+### Major Changes
+#### com.unity.ml-agents (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
-## [Unreleased]
+### Minor Changes
+#### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+### Bug Fixes
+#### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+## [1.9.0-preview] - 2021-03-17
 ### Major Changes
 #### com.unity.ml-agents (C#)
 - The `BufferSensor` and `BufferSensorComponent` have been added. They allow the Agent to observe variable number of entities. (#4909)