Add a ruby on rails guide

free-programming-books.md

@@ -1632,6 +1632,7 @@ See also [TeX](#tex)
 * [Railscasts - Ruby on Rails Screencasts](http://railscasts.com/)
 * [A community-driven Rails style guide](https://github.com/bbatsov/rails-style-guide)
 * [Kestrels, Quirky Birds, and Hopeless Egocentricity](https://leanpub.com/combinators/read)
+* [Learn Ruby on Rails as You Modify a Craigslist Clone](http://www.thinkful.com/learn/ruby-on-rails-tutorial/)
 * [Objects on Rails](http://objectsonrails.com)
 * [Rails Girls Guides](http://guides.railsgirls.com)
 * [Ruby on Rails 3.2 - Step by Step](http://www.xyzpub.com/en/ruby-on-rails/3.2/)