Merge pull request #4900 from Unity-Technologies/fix-readme

Adding a new line to Readme.md so the next line is not part of the table

README.md

 | **Release 8** | October 14, 2020 | [source](https://github.com/Unity-Technologies/ml-agents/tree/release_8) | [docs](https://github.com/Unity-Technologies/ml-agents/tree/release_8_docs/docs/Readme.md) | [download](https://github.com/Unity-Technologies/ml-agents/archive/release_8.zip) | [0.21.0](https://pypi.org/project/mlagents/0.21.0/) | [1.5.0](https://docs.unity3d.com/Packages/com.unity.ml-agents@1.5/manual/index.html) |
 | **Verified Package 1.0.5** | September 23, 2020 | [source](https://github.com/Unity-Technologies/ml-agents/tree/com.unity.ml-agents_1.0.5) | [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_1.0.5.zip) | [0.16.1](https://pypi.org/project/mlagents/0.16.1/) | [1.0.5](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) |
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 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).