postgresql setup guide for macOS

Related issues #45

User story As a mac user, I want a postgresql setup guide for mac so that I could install postgresql on my mac machine without pain

Acceptance criteria

Step by step guide to set up postgresql on mac machine
An entire guide or a link to the guide should be put in README.md at the proejct root so that it could be easily discovered by prospective users

Definition of Done I know this is done when I can find a detailed setup guide in Github README.md about how to install postgresql on mac machine

gcivil-nyu-org / spring2020-cs-gy-9223-backend