TOTP quickstart for github

[gl-yziquel]

Hi.

I stumbled on this git repository as GitHub is currently bullying me into using TOTP.

I unfortunately know next to nothing about TOTP.

A very quick start guide detailing or sketching how I should get the shared key that your totp cli begs from me when running totp add github would be more than welcome. Just a line or two.

It would have made my workflow much more painless, and would have eased the adoption of your tool.

(Context: I'm a cli and text based interface fanatic.)

[gl-yziquel]

Suggested PR: #23