Add docs badge to README

Hi José,

this patch is a rebased version of this one: https://github.com/elixir-lang/ecto/pull/302

It adds a docs badge to the README: [![Inline docs](http://inch-ci.org/github/elixir-lang/ecto.svg?branch=master&style=flat)](http://inch-ci.org/github/elixir-lang/ecto)

Thanks for considering!