Add docs badge to README

Hi Kostyantyn,

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/kostyantyn/hydra_attribute.png)](http://inch-pages.github.io/github/kostyantyn/hydra_attribute)

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 Hydra is http://inch-pages.github.io/github/kostyantyn/hydra_attribute/

Inch Pages is still in it's infancy, but already used by projects like [Guard](https://github.com/guard/guard), [Haml](https://github.com/haml/haml), [Pry](https://github.com/pry/pry), and [Libnotify](https://github.com/splattael/libnotify).

What do you think?