remove unreleased section of changelog (#4754)

com.unity.ml-agents/CHANGELOG.md

 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 
-## [Unreleased]
+## [1.7.0-preview] - 2020-12-21
 ### Major Changes
 #### com.unity.ml-agents (C#)
 #### ml-agents / ml-agents-envs / gym-unity (Python)