Documentation

I'm interested in helping with documentation.

**My initial proposal**

- Use https://readthedocs.org/ to host and Sphinx for auto documentation
- There seems to be a desire to keep the project as simple to contribute to as possible, so we would stick with .md instead .rst
- md files would stay in the root docs folder
- We may need to clean up docstrings but I can see what we can do about not requiring strict docstrings? https://github.com/jldbc/pybaseball/issues/125

Let me know if there's no desire to use Read the Docs