Add link to ReadTheDocs in GitHub Project Description

[johnthagen]

Add https://djangochannelsrestframework.readthedocs.io/en/latest/ to repo's description. E.g.:

This will make it easier for new users to quickly find the documentation as it's in a familiar/common location.

[Matthaus Woolard]

I have updated GH, was not aware this was a feature on GH.