developers: 2FA on GitHub

[Hilary Oliver]

To former and/or occasional Cylc  contributors:

Two-factor authentication is now a requirement for membership of the "cylc organisation" on GitHub. A bunch of past contributors got excluded when this was made compulsory yesterday. If you are one these people, just configure 2FA for your own GitHub account and I'll reinstate you by request.

  https://help.github.com/articles/securing-your-account-with-two-factor-authentication-2fa/

Thanks,

Hilary