Add date to changelog

com.unity.ml-agents/CHANGELOG.md

 and this project adheres to
 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
-## [1.9.1-preview]
+## [1.9.1-preview] - 2021-04-12
 ### Major Changes
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - The `--resume` flag now supports resuming experiments with additional reward providers or