Update changelog before branching for release 7 (#4468)

* update changelog before release

* missed section

com.unity.ml-agents/CHANGELOG.md

 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
-
+#### com.unity.ml-agents (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+### Bug Fixes
+#### com.unity.ml-agents (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+## [1.4.0-preview] - 2020-09-16
+### Major Changes
+#### com.unity.ml-agents (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+### Minor Changes
 #### com.unity.ml-agents (C#)
 - Update Barracuda to 1.1.0-preview (#4208)
 - Enabled C# formatting using `dotnet-format`. (#4362)