Add docs badge to README

Hi Justin,

this is the re-applied patch you asked for here: https://github.com/justinfrench/formtastic/pull/1000

I agree with the concerns you raised there. Especially regarding class- vs method-docs, but I have not come across a viable solution to this problem.

[Inch Pages](http://inch-pages.github.io) is supposed to be a project that tries to encourage aspiring Rubyists to document their code by showing that community veterans do it as well. The point should not be to achieve "100% docs" (which is why I don't use numbers in the badges).

I would be very happy if Formtastic would join this little movement!