Update readme for release_13. (#4951)

README.md

 
 ## Releases & Documentation
 
-**Our latest, stable release is `Release 12`. Click
+
+**Our latest, stable release is `Release 13`. Click
 [here](https://github.com/Unity-Technologies/ml-agents/tree/release_13_docs/docs/Readme.md)
 to get started with the latest release of ML-Agents.**
 
 | **Version** | **Release Date** | **Source** | **Documentation** | **Download** | **Python Package** | **Unity Package** |
 |:-------:|:------:|:-------------:|:-------:|:------------:|:------------:|:------------:|
 | **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 12** | **December 22, 2020** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/release_12)** | **[docs](https://github.com/Unity-Technologies/ml-agents/tree/release_12_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/release_12.zip)** | **[0.23.0](https://pypi.org/project/mlagents/0.23.0/)** | **[1.7.2](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.7/manual/index.html)** |
+| **Release 13** | **February 17, 2021** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/release_13)** | **[docs](https://github.com/Unity-Technologies/ml-agents/tree/release_13_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/release_13.zip)** | **[0.24.0](https://pypi.org/project/mlagents/0.24.0/)** | **[1.8.0](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.8/manual/index.html)** |
+| **Release 12** | December 22, 2020 | [source](https://github.com/Unity-Technologies/ml-agents/tree/release_12) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/release_12_docs/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/release_12.zip) | [0.23.0](https://pypi.org/project/mlagents/0.23.0/) | [1.7.2](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.7/manual/index.html) |
 | **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) | [0.23.0](https://pypi.org/project/mlagents/0.23.0/) | [1.7.0](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.7/manual/index.html) |
 | **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) | [0.22.0](https://pypi.org/project/mlagents/0.22.0/) | [1.6.0](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.6/manual/index.html) |
 | **Verified Package 1.0.6** | **November 16, 2020** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/com.unity.ml-agents_1.0.6)** | **[docs](https://github.com/Unity-Technologies/ml-agents/blob/release_2_verified_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/com.unity.ml-agents_1.0.6.zip)** | **[0.16.1](https://pypi.org/project/mlagents/0.16.1/)** | **[1.0.6](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.0/manual/index.html)** |
-| **Release 7** | September 16, 2020 | [source](https://github.com/Unity-Technologies/ml-agents/tree/release_7) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/release_7_docs/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/release_7.zip) | [0.20.0](https://pypi.org/project/mlagents/0.20.0/) | [1.4.0](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.4/manual/index.html) |
 
 If you are a researcher interested in a discussion of Unity as an AI platform,
 see a pre-print of our

utils/make_readme_table.py

     ReleaseInfo("release_10", "1.6.0", "0.22.0", "November 18, 2020"),
     ReleaseInfo("release_11", "1.7.0", "0.23.0", "December 21, 2020"),
     ReleaseInfo("release_12", "1.7.2", "0.23.0", "December 22, 2020"),
+    ReleaseInfo("release_13", "1.8.0", "0.24.0", "February 17, 2021"),
     # Verified releases
     ReleaseInfo("", "1.0.6", "0.16.1", "November 16, 2020", is_verified=True),
     ReleaseInfo("", "1.0.5", "0.16.1", "September 23, 2020", is_verified=True),