turn README.md to RST and input into docs index via "include"

[Stefan Appelhoff]

This would make it easier for us to keep consistent data between the README and our documentation --> only one place to maintain both.

for the include directive, see an example here: https://github.com/mne-tools/mne-bids/blob/master/doc/index.rst

a nice md to rst converter: https://github.com/miyakogi/m2r