Update readme for contributers

[imalsogreg]

Task

Document how to pull the dependencies and do a local build
Document how to serve the app locally
Document how to make various contributions, e.g.

• Where CSV data behind a map lives
• Where navigation lives

Acceptance Criteria

• New contributor can self-onboard with a little help from slack
• Following instructions verbatim results in a running local react server (or server for a static build)

Notes