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.
So, you've opened this sentence with "in some languages", and deliberately noted that this is Ruby code here (at least in the tag for the code block; hopefully the reader will also be able to tell that it is ruby and not rust...).

As a reader, I'm either confused or on the edge of my seat.  Sample thoughts: "Where is this going?  Is assignment an expression in Rust too?"

But there's no pay off here.  I cannot tell if you are planning to flesh this sentence and associated example out more later, or ...?

So I would suggest that you either:
 * remove this sentence and the ruby example, or
 * point out that Rust *is* a language where assignment is an expression, but the value produced by the assignment itself is not very useful.  (And then maybe point down to the discussion of "unit" below and expound more on that topic there.)

I recognize the second bullet is wand-wavy and potentially a lot of work.  So I recommend you go with the first bullet. :)