Add/Update Env Vars Description in Readme

[arruda]

This is a good first issue for anyone looking into contributing to our project!

We need to update in our Readme in the "Configure Environement Variables" section, the full updated list of the environment variables used in this service.
Each env var should be listed together with a short description (and if possible, some example of values) about it. For example, adding the DJANGO_DEBUG env var to this list, it would look like this:

• DJANGO_DEBUG: Sets the system into debug mode, can be True or False

To get the list of the environment variables in this project you'll need to look into this settings config file: gatekeeper/settings.py, and look for calls that look like any of these:

• DEBUG = os.getenv('DJANGO_DEBUG', 'False') == 'True' The env var here is: DJANGO_DEBUG
• SECRET_KEY = get_env_var('DJANGO_SECRET_KEY') The env var here is: DJANGO_SECRET_KEY.

Please do a single PR that includes all of these env vars and their description in the README file.

[Tosinibikunle]

I wiil like to work on this