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 have in our Readme in the "Setting up Configurations (.env file)" 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 LOGGING_LEVEL env var to this list, it would look like this:

• LOGGING_LEVEL: Defines the logging level of the system, for example: DEBUG

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

• JWT_LOCAL_USER_ID_FIELD = os.environ.get('JWT_LOCAL_USER_ID_FIELD', 'username') The env var here is: JWT_LOCAL_USER_ID_FIELD
• JWT_SIGNING_KEY = get_env_var('JWT_SIGNING_KEY') The env var here is: JWT_SIGNING_KEY.
• DEFAULT_API_VERSION = config('DEFAULT_API_VERSION', default='1.0.0') the env var here is DEFAULT_API_VERSION.

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