Writing the CONFIGURING.md doc.

:fork_and_knife:

This is still a work in progress. I want to explain the configuration a bit more in an isolated place outside of the README. Configuration of Supermarket takes a bit of time to setup, so I want to make sure that the docs for it are as straight-forward as possible.

todo:

- [x] expand on some of other .env settings
- [x] add info to the README that points to the doc