Add readthedocs site and support

[jameslamb]

This project should be documented on RTD!

This involves:

1. Add sphinx setup per #40
2. Add a .readthedocs.yml, e.g. https://github.com/jameslamb/doppel-cli/blob/master/.readthedocs.yml
3. @ntdef or @ngparas or I need to go to https://readthedocs.org/dashboard and set up the project