provide instructions for GitHub for Mac

[afeld]

I imagine a lot of (people we want to be) Guide creators won't be comfortable with the command line...would be good to provide a less intimidating path for them.

[wslack]

https://docs.google.com/document/d/18b-4VPTcuqat-enGQSVzivGH2CsqdQVG0K0eToRM39I/edit

I have screenshots on pages 9-10. Would be happy for someone to adapt this into a guide; was written originally for people to upload their pictures.