Hello cargo

Built on top of https://github.com/rust-lang/rust/pull/15181. https://github.com/steveklabnik/rust/commit/e52719291183d7b4f691a1997c69fd75b8101b07 is the only new commit, you may want to review that separately.

I'm not sure if the car analogy is too glib.
I think “subreddit” should be replaced with “the /r/rust subreddit” or “the rust subreddit” or “our subreddit”. (The word “our” can be inside or outside the link, preferably inside, and the one before the mailing list should match.)