Add documentation about crates.io usage

Rendered: http://alexcrichton.com/cargo/crates-io.html

cc @steveklabnik, @wycats 
I think this "from" can disappear, and I think the second clause could be rearranged slightly (trailing 'from' always sounds a bit weird to me).

> This site serves as a location to discover and download packages, and `cargo` is configured to use it by default.