Add Read Ruby.

free-programming-books.md

@@ -1568,6 +1568,7 @@ See also [TeX](#tex)
 * [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/)
+* [Read Ruby](http://readruby.io/)
 * [Ruby Essentials](http://www.techotopia.com/index.php/Ruby_Essentials)
 * [Ruby Hacking Guide](http://ruby-hacking-guide.github.io/)
 * [Ruby in Twenty Minutes](https://www.ruby-lang.org/en/documentation/quickstart/)