[docs] Update readme and changelog (#5107)

README.md

 ## Releases & Documentation
 
 
-**Our latest, stable release is `Release 14`. Click
+**Our latest, stable release is `Release 15`. Click
 [here](https://github.com/Unity-Technologies/ml-agents/tree/release_15_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 14** | **March 5, 2021** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/release_14)** | **[docs](https://github.com/Unity-Technologies/ml-agents/tree/release_14_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/release_14.zip)** | **[0.24.1](https://pypi.org/project/mlagents/0.24.1/)** | **[1.8.1](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.8/manual/index.html)** |
+| **Release 15** | **March 17, 2021** | **[source](https://github.com/Unity-Technologies/ml-agents/tree/release_15)** | **[docs](https://github.com/Unity-Technologies/ml-agents/tree/release_15_docs/docs/Readme.md)** | **[download](https://github.com/Unity-Technologies/ml-agents/archive/release_15.zip)** | **[0.25.0](https://pypi.org/project/mlagents/0.25.0/)** | **[1.9.0](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.9/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 14** | March 5, 2021 | [source](https://github.com/Unity-Technologies/ml-agents/tree/release_14) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/release_14_docs/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/release_14.zip) | [0.24.1](https://pypi.org/project/mlagents/0.24.1/) | [1.8.1](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.8/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) |

com.unity.ml-agents/CHANGELOG.md

 and this project adheres to
 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
-### Major Changes
-#### com.unity.ml-agents (C#)
-#### ml-agents / ml-agents-envs / gym-unity (Python)
-
-### 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 / com.unity.ml-agents.extensions (C#)
-#### ml-agents / ml-agents-envs / gym-unity (Python)
-
 ## [1.9.0-preview] - 2021-03-17
 ### Major Changes
 #### com.unity.ml-agents (C#)