Add docs badge to README

Hi there,

I noticed that you looked at Inch CI a while ago, but I had a bug in my server that prevented your status page from being build.

[![Inline docs](http://inch-ci.org/github/sporkmonger/addressable.png)](http://inch-ci.org/github/sporkmonger/addressable)

The badge - as you might know - 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/sporkmonger/addressable/

I just wanted to point out that it works now. If you are interested in sporting this badge in your README, you can accept this PR, if not, no hurt feelings. :smiley: