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.
Okay, so now that I'm done nit-picking the dialogue here, back to the matter at hand:

Perhaps this problem could be resolved by just saying:
"In some languages, new bindings can be introduced via subexpressions, rather than being isolated to statements. Like Ruby: <Ruby example>.  But Rust is not such a language."