Guide skeleton

This diff will look better once bors takes care of https://github.com/rust-lang/rust/pull/15183

@brson and I talked about it, and, if I commit this skeleton, I can submit PRs for each portion, without doing this silly "builds on previous PRs" stuff, and it shouldn't cause conflicts.

This lays out what I think the guide should cover, and in what order.  I haven't picked a cohesive project yet that shows all this off, but I think this progression of concepts is appropriate.