Add Practicing Ruby as a resource

Although not technically a book, these resources on how Ruby works is very detailed and a good way to dig deeper into Ruby.

free-programming-books.md

@@ -1435,6 +1435,7 @@ See also [TeX](#tex)
 * [Learn Ruby the hard way](http://ruby.learncodethehardway.org/book/)
 * [Learn to Program, by Chris Pine](http://pine.fm/LearnToProgram/)
 * [Mr. Neighborly's Humble Little Ruby Book](http://www.humblelittlerubybook.com/)
+* [Practicing Ruby](https://github.com/elm-city-craftworks/practicing-ruby-manuscripts)
 * [Programming Ruby](http://www.ruby-doc.org/docs/ProgrammingRuby/)
 * [Ruby Essentials](http://www.techotopia.com/index.php/Ruby_Essentials)
 * [Ruby Hacking Guide](http://ruby-hacking-guide.github.io/)