Guide: if

Adding the section about if to the guide. I hope this won't conflict with https://github.com/rust-lang/rust/pull/15229 , but we'll just have to see, I guess.
> Rust is not, according to the manual

@steveklabnik What?

 1. http://doc.rust-lang.org/rust.html#assignment-expressions

 2. Example of assignment as an expression: [playpen](http://play.rust-lang.org/?code=fn%20main%28%29%20{%0A%20%20%20%20let%20mut%20y%20%3D%200u%3B%0A%20%20%20%20println!%28%22y_1%3A%20{}%22%2C%20y%29%3B%0A%20%20%20%20let%20z%20%3D%20y%20%3D%201%3B%0A%20%20%20%20println!%28%22y_2%3A%20{}%22%2C%20y%29%3B%0A%20%20%20%20println!%28%22z%3A%20{}%22%2C%20z%29%3B%0A})