[Release 11]Update unreleased section in changelog (#4751)

com.unity.ml-agents/CHANGELOG.md

 #### com.unity.ml-agents (C#)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - TensorFlow trainers have been removed, please use the Torch trainers instead. (#4707)
+
+### 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 (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+
+## [1.7.0-preview] - 2020-12-21
+### Major Changes
+#### com.unity.ml-agents (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
 - PyTorch trainers now support training agents with both continuous and discrete action spaces. (#4702)
 ### Minor Changes
 #### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)