move extensions changelog entries to main changelog (#4427)

com.unity.ml-agents.extensions/CHANGELOG.md

 # Changelog
-All notable changes to this package will be documented in this file.
-
-The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
-and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+This changelog isn't used; please add to the `com.unity.ml-agents` changelog instead.
-
- ### Minor Changes
-#### com.unity.ml-agents.extensions (C#)
-- Added the GridSensor and GridSensor tests
-#### ml-agents-envs  (Python)
-- Modified the rpc_utils.py to support multiple PNGs

com.unity.ml-agents/CHANGELOG.md

 #### ml-agents / ml-agents-envs / gym-unity (Python)
 
 ### Minor Changes
+
+- GridSensor was added to the com.unity.ml-agents.extensions package. Thank you
+to Jaden Travnik from Eidos Montreal for the contribution! (#4399)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - Experimental PyTorch support has been added. Use `--torch` when running `mlagents-learn`, or add
 `framework: pytorch` to your trainer configuration (under the behavior name) to enable it.
+- Compressed visual observations with >3 channels are now supported. In
+ISensor.GetCompressedObservation(), this can be done by writing 3 channels at a
+time to a PNG and concatenating the resulting bytes. (#4399)
 - A CNN (`vis_encode_type: match3`) for smaller grids, e.g. board games, has been added.
 (#4434)