Add git & GitHub resources to README

[KirstieJane]

• Link to MozWOW resources.
• Set expectations
  • Make sure to explain philosophy of "Just do it" rather than learning everything from first principles.

[KirstieJane]

https://mozilla.github.io/open-leadership-training-series/articles/github-for-collaboration/

[KirstieJane]

https://github.com/oliviaguest/git-and-github-cheat-sheet

[KirstieJane]

http://rik.smith-unna.com/command_line_bootcamp