Add Rails layout description

hi.  thanks for moving forward with this.  

i was just about to try it out, but i didn't see the overall rails file layout description anywhere obvious in the docs.  i presume this is a drop-in replacement for https://github.com/codebrew/backbone-rails, so everything ought to be identical --  so i blindly copied the info from https://raw.github.com/codebrew/backbone-rails/master/README.md

i don't know if you have a better place you'd like to put this info (or it's already somewhere i didn't see). if so of course ignore this PR.  but i do figure this info ought to be *somewhere* easy to find, for somebody coming to railsy_backbone without starting with backbone-rails first)

cheers