Update changelog for release 13. (#4938)

Author: surfnerd

com.unity.ml-agents/CHANGELOG.md

@@ -6,11 +6,24 @@ 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 (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+
+## [1.8.0-preview] - 2021-02-17
+### Major Changes
+#### com.unity.ml-agents (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
 - TensorFlow trainers have been removed, please use the Torch trainers instead. (#4707)
 - A plugin system for `mlagents-learn` has been added. You can now define custom
   `StatsWriter` implementations and register them to be called during training.