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/justinfrench/formtastic.png)](http://inch-pages.github.io/github/justinfrench/formtastic)

It 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 Formtastic is http://inch-pages.github.io/github/justinfrench/formtastic/

Inch Pages is still in it's infancy, but already used by projects like [Reek](https://github.com/troessner/reek) and [libnotify](https://github.com/splattael/libnotify).

What do you think?