Add a bare-bones index.rst to fix readthedocs build

[therefromhere]

I presume either readthedocs or sphinx used to look for README.rst (3 years ago heh), and doesn't any more. Resolved by adding in a bare-bones index.rst.

Resolves #82

Also:

• Check if the github webhooks are still working, since I notice you merged a PR yesterday and it doesn't seem to have caused a (failed) readthedocs build?
• I suggest changing the python interpreter in the readthedocs config to 3.x.

[coveralls]

Coverage remained the same at 0.0% when pulling 39974f36e1261f0641a1738c0ac88e20183c8743 on therefromhere:rtd_fix into 7cab78c4531780ab4b32033d5104ccd5be1a246a on ierror:master.