Update table with new release (#5425)

README.md

 
 ## Releases & Documentation
 
-**Our latest, stable release is `Release 17`. Click
+**Our latest, stable release is `Release 18`. Click
 [here](https://github.com/Unity-Technologies/ml-agents/tree/release_18_docs/docs/Readme.md)
 to get started with the latest release of ML-Agents.**
 
 | **Version** | **Release Date** | **Source** | **Documentation** | **Download** | **Python Package** | **Unity Package** |
 |:-------:|:------:|:-------------:|:-------:|:------------:|:------------:|:------------:|
 | **main (unstable)** | -- | [source](https://github.com/Unity-Technologies/ml-agents/tree/main) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/main/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/main.zip) | -- | -- |
+| **Release 18** | **June 9, 2021** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/release_18)** | **[docs](https://github.com/Unity-Technologies/ml-agents/tree/release_18_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/release_18.zip)** | **[0.27.0](https://pypi.org/project/mlagents/0.27.0/)** | **[2.1.0](https://docs.unity3d.com/Packages/com.unity.ml-agents@2.1/manual/index.html)** |
-| **Release 17** | **April 22, 2021** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/release_17)** | **[docs](https://github.com/Unity-Technologies/ml-agents/tree/release_17_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/release_17.zip)** | **[0.26.0](https://pypi.org/project/mlagents/0.26.0/)** | **[2.0.0](https://docs.unity3d.com/Packages/com.unity.ml-agents@2.0/manual/index.html)** |
+| **Release 17** | April 22, 2021 | [source](https://github.com/Unity-Technologies/ml-agents/tree/release_17) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/release_17_docs/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/release_17.zip) | [0.26.0](https://pypi.org/project/mlagents/0.26.0/) | [2.0.0](https://docs.unity3d.com/Packages/com.unity.ml-agents@2.0/manual/index.html) |
-
 If you are a researcher interested in a discussion of Unity as an AI platform,
 see a pre-print of our
 [reference paper on Unity and the ML-Agents Toolkit](https://arxiv.org/abs/1809.02627).

utils/make_readme_table.py

     ReleaseInfo("release_15", "1.9.0", "0.25.0", "March 17, 2021"),
     ReleaseInfo("release_16", "1.9.1", "0.25.1", "April 13, 2021"),
     ReleaseInfo("release_17", "2.0.0", "0.26.0", "April 22, 2021"),
+    ReleaseInfo("release_18", "2.1.0", "0.27.0", "June 9, 2021"),
     # Verified releases
     ReleaseInfo("", "1.0.8", "0.16.1", "May 26, 2021", is_verified=True),
     ReleaseInfo("", "1.0.7", "0.16.1", "March 8, 2021", is_verified=True),