Add inline documentation

Inline documentation was purposely left off when things were changing, but now the API is starting to stabilize. Adding documentation to the public facing methods would be fantastic.

If you'd like to help, here is how Crystal docs work: https://crystal-lang.org/docs/conventions/documenting_code.html

Feel free to make small PRs so it's easy to review and merge them in :D

Thanks so much for the help!