Remove "extern crate" directions from README

[zmitchell]

You no longer need to add extern crate directives for modern Rust editions.

[JohnTitor]

Thanks! Our MSRV is, however, 1.23.0 and it's still needed on such an old version. Maybe we should mention it instead of just removing, like:

If you're using Rust 1.30 or earlier, or edition 2015, add this to your crate root:

[zmitchell]

Done