Add docs badge to README

Hi there,

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/Shopify/liquid.png)](http://inch-pages.github.io/github/Shopify/liquid)

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/Shopify/liquid/

Inch Pages is still a young project, 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 [ROM](https://github.com/rom-rb/rom).

What do you think?