update readme table and README (#4757)

README.md

 
 ## Releases & Documentation
 
-**Our latest, stable release is `Release 10`. Click
+**Our latest, stable release is `Release 11`. Click
 [here](https://github.com/Unity-Technologies/ml-agents/tree/release_11_docs/docs/Readme.md)
 to get started with the latest release of ML-Agents.**
 
 | **Version** | **Release Date** | **Source** | **Documentation** | **Download** |
 |:-------:|:------:|:-------------:|:-------:|:------------:|
 | **master (unstable)** | -- | [source](https://github.com/Unity-Technologies/ml-agents/tree/master) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/master/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/master.zip) |
-| **Release 10** | **November 18, 2020** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/release_11)** | **[docs](https://github.com/Unity-Technologies/ml-agents/tree/release_11_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/release_11.zip)** |
+| **Release 11** | **December 21, 2020** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/release_11)** | **[docs](https://github.com/Unity-Technologies/ml-agents/tree/release_11_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/release_11.zip)** |
+| **Release 10** | November 18, 2020 | [source](https://github.com/Unity-Technologies/ml-agents/tree/release_10) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/release_10_docs/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/release_10.zip) |
-| **Release 4** | July 15, 2020 | [source](https://github.com/Unity-Technologies/ml-agents/tree/release_4) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/release_4_docs/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/release_4.zip) |
 
 ## Citation
 

utils/make_readme_table.py

     ReleaseInfo("release_8", "1.5.0", "0.21.0", "October 14, 2020"),
     ReleaseInfo("release_9", "1.5.0", "0.21.1", "November 4, 2020"),
     ReleaseInfo("release_10", "1.6.0", "0.22.0", "November 18, 2020"),
+    ReleaseInfo("release_11", "1.7.0", "0.23.0", "December 21, 2020"),
 ]
 
 MAX_DAYS = 150  # do not print releases older than this many days