Add some entries to Release 16 changelog (#5237)

* Update changelog

* Update com.unity.ml-agents/CHANGELOG.md

Co-authored-by: Vincent-Pierre BERGES <vincentpierre@unity3d.com>

* Fix grammar

Co-authored-by: Vincent-Pierre BERGES <vincentpierre@unity3d.com>

com.unity.ml-agents/CHANGELOG.md

 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - The `--resume` flag now supports resuming experiments with additional reward providers or
  loading partial models if the network architecture has changed. See
- [here](https://github.com/Unity-Technologies/ml-agents/blob/main/docs/Training-ML-Agents.md#loading-an-existing-model)
+ [here](https://github.com/Unity-Technologies/ml-agents/blob/release-16_docs/docs/Training-ML-Agents.md#loading-an-existing-model)
+
+#### com.unity.ml-agents (C#)
+- Fixed erroneous warnings when using the Demonstration Recorder. (#5216)
+
+- ELO now correctly resumes when loading from a checkpoint. (#5202)
+- In the Python API, fixed `validate_action` to expect the right dimensions when `set_action_single_agent` is called. (#5208)
+- In the `GymToUnityWrapper`, raise an appropriate warning if `step()` is called after an environment is done. (#5204)
+- Fixed an issue where using one of the `gym` wrappers would override user-set log levels. (#5201)
 
 ## [1.9.0-preview] - 2021-03-17
 ### Major Changes