Initial stab at writing docs

Used sphinx so the docs could be hosted at Read the Docs.

Closes #41 and #54 .
We should explain this a bit more, e.g. "Python-modernize doesn't include alternatives for these fixers, so using this option will leave some compatibility problems unaddressed. Even if you wish to avoid an external dependency, it's probably easier to run all the fixers, and then replace references to `six` with your own compatibility module".

I'd lose the 'strongly' - saying 'not recommended' is fine, but there's no need to scare people. It's not like that flag is a security risk or something.