Add git workflow instructions to readme

[aaronsn]

• Describe forking workflow
• Suggest using "Squash and merge"
• Explain downsides to "Squash and merge" and how to do dependent changes.

[aaronsn]

Sorry the merge got messed up when I tried to pull in your readme changes. Made a new PR here: https://github.com/SatcherInstitute/prototype/pull/50 and addressed your comments there