Add documentation about crates.io usage

Rendered: http://alexcrichton.com/cargo/crates-io.html

cc @steveklabnik, @wycats 
Maybe this could use concrete numbers ("`0.1` and `0.2` ... but `1.1` and `1.2`") or parens ("`0.x` and `0.(x+1)` ... but `1.x` and `1.(x+1)`").