Add docs badge to README

Hi Pavel,

this patch adds a docs badge to the README to show off inline-documentation to potential contributors: [![Inline docs](http://inch-ci.org/github/rwz/nestive.png)](http://inch-ci.org/github/rwz/nestive)

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/rwz/nestive/

But I guess you know that already, since Mock5 has an Inch CI badge ... Well, I would be glad if Nestive joined my little "pro documentation" movement as well :wink: