Add Github SSH Keys

[ajxchapman]

SSH private keys can be tested against github.com to see if they are registered against an existing user account. If the key exists the username corresponding to the key will be provided:

$ ssh -i <path to SSH private key> -T git@github.com
Hi <username>! You've successfully authenticated, but GitHub does not provide shell access.

[EdOverflow]

Thank you for your contribution, @ajxchapman.

[zzeitlin]

The same is true for gitlab.com:

ssh -i <path to SSH private key> -T git@gitlab.com
Welcome to GitLab, <username>!