Added instructions for how to contribute

I removed the README.md file because it did not have anything useful in it and we might as well keep the documentation files in RST.  I created a README.rst file in the root directory and added details for how to fork and contribute to the documentation.  I also modified the README.rst file in the `rtd` directory to refer to the root README.rst file for how to contribute to the documentation.

I think it is important to have a README in the root with contribution instructions so users see it right away when they navigate to the repo page on GitHub.