Add post on GitHub pro tips

Sharing some pro tips that some of us might not know. 
Used for a lunch and learn in the US office
I know it's already public, but I'm not so sure it's good form to "out" people this way.

Though, https://github.com/johndouthat/mydotfiles/blob/81b657398c71c33c6b7459c5aa8c2f070adadca9/.ssh/id_rsa I LOL'ed