Modifying the changelog for release 17 (#5277)

* Modifying the changelog for release 17

* Change version to 2.0.0-exp.1

* Removing the [Unreleased] section

com.unity.ml-agents/CHANGELOG.md

 and this project adheres to
 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
+
+## [2.0.0-exp.1] - 2021-04-22
 ### Major Changes
 #### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
 - The minimum supported Unity version was updated to 2019.4. (#5166)