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/rom-rb/rom.png)](http://inch-pages.github.io/github/rom-rb/rom) (regarding your integration of yardstick into the workflow, the completeness comes as no suprise :wink: )

Although I already spoke to some of you, let me leave my usual note about this badge:

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 ROM is http://inch-pages.github.io/github/rom-rb/rom/

Inch Pages is still in it's infancy, but already used by projects like [Guard](https://github.com/guard/guard) and [Pry](https://github.com/pry/pry).

Hope you like it!