add rustonomicon

free-programming-books.md

@@ -1923,6 +1923,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 * [Rust by Example](http://rustbyexample.com)
 * [Rust for Rubyists](http://www.rustforrubyists.com/book/index.html)
 * [The Rust Programming Language](http://doc.rust-lang.org/book/)
+* [The Rustonomicon](https://doc.rust-lang.org/nomicon/)
 * [Why Rust?](http://www.oreilly.com/programming/free/files/why-rust.pdf) (PDF)