Add documentation about crates.io usage
-l flag no longer pass to binaries if there's a library in the package
auto merge of #864 : tomaka/cargo/fix-814, r=alexcrichton

Close #814 

Unfortunately I don't really know how I'd add a test for this.
In order to try compile a library then a binary, you need to successfully compile the library, which means that you'd need to pass a valid value instead of just `-l foo`
auto merge of #854 : alexcrichton/cargo/crates-io-dox, r=steveklabnik

Rendered: http://alexcrichton.com/cargo/crates-io.html

cc @steveklabnik, @wycats