Update changelog before Release 18 branch (#5412)

com.unity.ml-agents/CHANGELOG.md

 
 ## [Unreleased]
 ### Major Changes
+#### com.unity.ml-agents / com.unity.ml-agents.extensions (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 / com.unity.ml-agents.extensions (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+
+## [2.1.0-exp.1] - 2021-06-09
 ### Minor Changes
 #### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
 - update Barracuda to 2.0.0-pre.3. (#5385)
 - Colab notebooks illustrating the use of the Python API are now part of the repository. (#5399)
 ### Bug Fixes
-- The calculation of the target entropy of SAC with continuous actions was incorrect and has been fixed. (#5372)
+#### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+- The calculation of the target entropy of SAC with continuous actions was incorrect and has been fixed. (#5372)
 - Fixed an issue where the histogram stats would not be reported correctly in TensorBoard. (#5410)