Add some documentation for two-factor auth

[Blaisorblade]

First, thanks for this package!

Figuring out support for two-factor authentication (2FA) wasn't trivial. Since there isn't a wizard where to paste a token and check it's correct, at least add some summarized docs, specialized for magithub—since Ghub's docs can't contain client-specific information.

Notes:

• I don't like this solution, I just like it better than just filing an issue.

• Feel free to move that info to a 2FA subsection, but the link belongs to the intro docs since 2FA shouldn't be an obscure feature, but part of the "recommended" setup.

Security

I'm also not sure what's adequate security for the token. Ghub's docs only say

Secrets are stored in ~/.authinfo in plain text. If you don’t want that (good choice), then you have to customize auth-sources, e.g. by flipping the positions of the first two elements.

but I don't have GPG setup at all, let alone setup with Emacs and I don't plan to shave that yak yet — customizing auth-sources can't possibly be enough?

EDITED: So I guess that for users without 2FA tokens do get written in cleartext in ~/.authinfo? Guess that's unavoidable until support for keystores is automated enough, and a tolerable compromise on "secure enough" client machines.

[Blaisorblade]

Build failures https://travis-ci.org/vermiculus/magithub/jobs/345908758#L1070 and https://travis-ci.org/vermiculus/magithub/jobs/345908759#L1080 appear unrelated to my PR (right?), so please don't hold them against it.

[vermiculus]

Re Travis builds: yeah, it does that sometimes. It's supremely annoying, but there's an open ticket about it somewhere on debbugs I think.

[vermiculus]

Related: magit/ghub#47

[vermiculus]

Hard-wrapped and used Org syntax. Thanks!