[docs] Update changelog (#5095)

Author: Ervin T

com.unity.ml-agents/CHANGELOG.md

@@ -6,10 +6,22 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 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)
+
+### 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)
 - The `SimpleMultiAgentGroup` class and `IMultiAgentGroup` interface have been added. These allow Agents to be given rewards and
   end episodes in groups. (#4923)