Make cheat sheet for github usage

[divilian]

Include:

• clone
• add
• commit
• commit -a
• push
• pull
• diff (or difftool) (point to p.223 of Blueprints)
• status
• log

[divilian]

Do @vgcagle and @akochans still think this would be useful?

[akochans]

For right now I think I am good!

[vgcagle]

I'm good too, I can always use the two chapters of Blueprints as a reference.

[divilian]

Closed for now since my awesome students are too smart and self-motivated to even need this.