Document the need for personal tokens when using 2 factor authentication

Linked to #28.

If you have 2 factor authentication (2FA) turned on, you'll need to use a [personal token](https://help.github.com/articles/creating-an-access-token-for-command-line-use/) **instead of a password** if you **log into git from the command line**.

We need to have this documented somewhere if we plan to encourage 2FA.