Add a help file

It'd be nice if we could add a `docs/` folder with documentation that's accessible from within vim via `:h`. @moll have you thought about adding this?

I know there a few utilities out there for auto-generating documentation from comments (which are really easy to use, in my experience). If you'd be interested, I could try a PR using Google's [vimdoc](https://github.com/google/vimdoc) package.
