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-ci.org/github/causes/overcommit.svg)](http://inch-ci.org/github/causes/overcommit)

The badge links to [Inch CI](http://inch-ci.org), 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-ci.org/github/causes/overcommit/

What do you think?