Add "Developing Games With Ruby"

A free to read online ebook: https://leanpub.com/developing-games-with-ruby/read

free-programming-books.md

@@ -1567,6 +1567,7 @@ See also [TeX](#tex)
 ### Ruby
 * [A community-driven Ruby style guide](https://github.com/bbatsov/ruby-style-guide)
 * [CodeCademy Ruby](http://www.codecademy.com/tracks/ruby)
+* [Developing Games With Ruby](https://leanpub.com/developing-games-with-ruby/read)
 * [I Love Ruby](http://goo.gl/1qrMUN)
 * [Just Enough Ruby to Get By](http://dmtri.com/posts/65)
 * [Learn Ruby the hard way](http://ruby.learncodethehardway.org/book/)