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.
While the code is explained after this sentence, it seems a bit confusing at first. One probably doesn't get what "projects directory" means until they see the code. Perhaps something like "Go to where you keep your projects in"?