Add docs badge to README

Hi there,

I would like to propose a docs badge for Elixir projects. Since Elixir declared documentation to be a "first class citizen" and I already had created [Inch](http://trivelop.de/inch) for Ruby, I decided to try to implement "inline-docs linting" for Elixir as well.

This is what it would look like: [![Inline docs](http://inch-ci.org/github/phoenixframework/phoenix.svg)](http://inch-ci.org/github/phoenixframework/phoenix) (click to view status page)

The idea is to show to potential contributors that a lib/package is documented and to inspire people starting with Elixir to follow suit. The badge links to [Inch CI](http://inch-ci.org), a project that tries to raise the visibility of inline-docs to encourage aspiring programmers to document their code.  Inch CI is still relatively new (4 months old), but it is already relatively succesful (badges used by 300+ Ruby projects including [Guard](https://github.com/guard/guard), [Haml](https://github.com/haml/haml), [Pry](https://github.com/pry/pry), [ROM](https://github.com/rom-rb/rom), and [Sass](https://github.com/sass/sass)).

Please understand this not as "I built something, now use it!" but rather a call for discussion. I made a functioning prototype implementation of this (as the badge shows), but I want to emphazise the "prototype" part. There is no well-thought-out UI or workflow for you to add your own projects (yet) and the evaluation criteria are up for discussion as well as any other design choices I made.

@all: What do you think? Let's talk!