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.
PL nerd nit: languages like Perl and Ruby may conflate the syntax for introducing a new variable binding with the syntax for assigning a new value to an existing variable binding, but *not all* expression-based languages do so.  (e.g. Scheme is an easy counter-example.)

So it is incorrect to claim that in expression based languages, they are the same (where "they" here refers to binding (`let x = ...`) and assignment (`x = ...`).