Write a more helpful README

[crawfoal]

Give instructions on how to run through the demo, and only link to the production version (don't give staging link).

[crawfoal]

Include instructions for new developers, like:

• links on how to set up their machine for rails development
• how to setup their machine for development on this specific project, including how to run tests
• link to a contributing doc
• description review app / pull request workflow (maybe put this in the contributing doc instead):
  1. open pull request
  2. go to the project's heroku pipepline dashboard (can they actually access this?) and build the review app (should see option under review apps col)
  3. run the review app script: ./bin/review_app_setup (explain what that script does and why we don't just use the db:populate task)