Add docs badge to README

Hi there,

this patch adds a docs badge to the README to show off inline-documentation to the casual visitor: [![Inline docs](http://inch-pages.github.io/github/haml/haml.png)](http://inch-pages.github.io/github/haml/haml) (and Haml's inline documentation is really good!)

The badge links to [Inch Pages](http://inch-pages.github.io), a project that tries to raise the visibility of documentation in Ruby projects. The status page for Haml is http://inch-pages.github.io/github/haml/haml/

Inch Pages is still in it's infancy, but already used by projects like [Virtus](https://github.com/solnic/virtus) and [libnotify](https://github.com/splattael/libnotify).

What do you think?