Add docs badge to README

Hi Markus,

this patch adds a docs badge to the README to show off inline-documentation to potential contributors: [![Inline docs](http://inch-pages.github.io/github/mbj/mutant.png)](http://inch-pages.github.io/github/mbj/mutant)

The badge links to [Inch Pages](http://inch-pages.github.io), a project that tries to raise the visibility of inline-docs to encourage aspiring Rubyists to document their code. Your status page is http://inch-pages.github.io/github/mbj/mutant/

Inch Pages is still a young project, but already used by projects like [ROM](https://github.com/rom-rb/rom) and [Virtus](https://github.com/solnic/virtus), not to mention [Guard](https://github.com/guard/guard), [Haml](https://github.com/haml/haml), and [Pry](https://github.com/pry/pry).

What do you think?