Document opt-outs w/ mailkick

[joshuap]

We don't have built-in opt-outs, but it's super easy to add with the mailkick gem.

A related note to self: I'm wondering if there's a way to customize the Campaigns::Base#user SQL from the Rails application/campaign so that I can optionally exclude unsubscribed users in scheduler queries. The difference there vs. segment is that they won't move through campaigns at all if they're unsubscribed, and if they resubscribe they'd pick up exactly where they left off.