Add documentation

Fixes #2.

Badges (taken from [guard](https://github.com/guard/guard)):

- [ ] [Version](http://badge.fury.io)
- [ ] [Travis CI](https://travis-ci.org)
- [ ] [Gemnasium](https://gemnasium.com)
- [ ] [Code Climate](https://codeclimate.com)
- [ ] [Coveralls](https://coveralls.io)

Other things to do:

- [ ] Add note about [semantic versioning](http://semver.org)
- [ ] Fill out [gemspec](http://guides.rubygems.org/specification-reference/)