Add documentation about crates.io usage

Rendered: http://alexcrichton.com/cargo/crates-io.html

cc @steveklabnik, @wycats 
I wanted to do that yeah, but linking to a `master` version is likely to go out of date, and linking to a specific rev is also likely to go out of date, so I figured I wouldn't do it for now :(. What we really need is to host those docs somewhere!