Use `cargo add` when using external crates

[carols10cents]

trafficstars

Maybe giving a copy-pasteable cargo add rand@version command would prevent issues we've seen in the past when a newer version of rand is available with breaking changes, and people install the latest version instead of the version in the book?

Anyway, cargo add is nice and we should show it off now that it's stable!

[chriskrycho]

Relevant: #3331.

[carols10cents]

AHA nice! I thought I remembered something but thought it was an issue. I should have searched PRs too. Thank you!