Readme changes

[wpears]

The README needs quite a few updates.

• Setup needs to be clarified, specifically with regard to setting up postgres/postgis. The readme doesn't say anything about getting a database up and running, but this is a requirement for running the data setup scripts.
• The purpose/scope of this repository needs to be clarified. It's unclear from the readme what the code/data in the repository does. Running through the setup will get the data setup for API calls, but nothing is said about how to setup the app itself (both front and back-end).
• See proposed changes in https://github.com/wpears/mapusaurus/blob/f56cf87c86ec4105fa09b5e0f3af8ea74798ca6d/notes.txt (mostly incorporated)

[wpears]

There also needs to be some info on what postgis plugins/templates/addons are needed.

[wpears]

A reference to API.md in the readme would be helpful