Use README.md to write index.rst in documentation

[bobleesj]

The original README.md before cookiecutting has lots of useful information while there is no index.rst provided in the documentation.

We shall use the information to write the index.rst file.

[sbillinge]

yes, please do take care of blending the info from the old EADME.md into the new README.rst, and also as you mention, this can be reused in the docs and the long_description in pypi/conda-forge