rdoc documentation

[ellnix]

This gem doesn't really have great documentation. While there was discussion about the wiki in #541 I think rdoc documentation in the source code would be a better solution.

• would work offline
• could be picked up by code editors
• would be easier to keep up to date (if a pr makes a change it should change the documentation as well)

[ellnix]

@brunoocasali let me know if this is a good idea or we make efforts toward keeping an up to date wiki.

I'll start work on whichever you prefer.