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 a language where assignment is an expression,

Rust is _not_, according to the manual: http://doc.rust-lang.org/rust.html#item-declarations

Anywho, I think I know what I did here. Why not just swap the two examples? That way, it reads "In some languages, assignment is an expression: (and maybe C rather than Ruby here?) But in Rust, it is not:" ?