Added one Rust book

free-programming-books.md

@@ -545,6 +545,9 @@
 
 * [Ruby on Rails Tutorial: Learn Rails By Example](http://ruby.railstutorial.org/ruby-on-rails-tutorial-book)
 
+###Rust
+* [Rust for Rubyists](http://www.rustforrubyists.com/book/index.html)
+
 
 ###Scala