Guide: Variable bindings

Whew. So much here! Feedback very welcome.

This is the first part where we actually start learning things. I'd like to think I struck a good balance of explaining enough details, without getting too bogged down, and without being confusing... but of course I'd think that. :wink: 

As I mention in the commit comment, We probably want to move the guessing game to the rust-lang org, rather than just having it on my GitHub. Or, I could put the code inline. I think it'd be neat to have it as a project, so people can pull it down with Cargo. Until we make that decision, I'll just leave this here.
Agreed, you should use "After the above expression is evaluated".

"This" is ambiguous because it technically does refer to itself as an expression (or just a sentence). "That" is more technically correct, but isn't used frequently in examples as @TheHydroImpulse already mentioned. I think "the above" is going to be the best combination of unambiguity and frequency of use.