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.
while I'm here: I now agree with you that in the context of Ruby, the word "assignment" correctly describes the binding in the expressions below.  However, we are in the context of a Rust tutorial, not Ruby.  Therefore, I would recommend that you avoid using the word assignment except in the manner that it is used in the rest of Rust (namely, assignment expressions).

In other words, I would prefer if you rephrased the above as "In some languages, variable bindings can be written as expressions, not just statements.  Like Ruby:"