Added a link to Github’s 2-factor help.
Added a link to Github’s 2-factor help.

Safari-FIDO-U2F/Safari-FIDO-U2F,Safari-FIDO-U2F/Safari-FIDO-U2F,Safari-FIDO-U2F/Safari-FIDO-U2F,Safari-FIDO-U2F/Safari-FIDO-U2F