add a ruby book (EbookFoundation#2950)

Author: Alexey Gorelov

free-programming-books.md

@@ -2242,6 +2242,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Ruby Programming](http://www.linuxtopia.org/online_books/programming_books/ruby_tutorial/)
 * [Ruby User's Guide](http://www.rubyist.net/~slagell/ruby/)
 * [Ruby Web Dev: The Other Way](https://leanpub.com/rwdtow/read) - Yevhen Kuzminov
+* [Rubyfu](https://rubyfu.net)
 * [The Bastards Book of Ruby](http://ruby.bastardsbook.com)
 * [The Unofficial Ruby Usage Guide (at Google)](http://www.caliban.org/ruby/rubyguide.shtml) - Ian Macdonald
 * [Why's (Poignant) Guide to Ruby](http://poignant.guide)