First Time Setup Documentation: Update Readme with env vars and their use

[codecakes]

We do not current have enough documentation on the number of environment variables we use and why, in the readme for first timers.

Here are the initial env vars we use:

REACT_APP_GOOGLE_MAPS_API_KEY # Use to call maps api from google to render maps and places.
REACT_APP_UBER_CLIENT_ID # used for calling uber functions for navigation
REACT_APP_COOKIEBOT_SRC # used for cookie consent and tracking on our production website.
REACT_APP_COOKIEBOT_CONSENT_DECLARATION # a cookie bot consent src code that renders the declaration modal which allows users to select/deselect which cookies they consent to.
REACT_APP_COOKIEBOT_CONSENT_HEAD = "https://consent.cookiebot.com/uc.js"
REACT_APP_COOKIEBOT_CONSENT_HEAD_DATA_CBID # cookiebot header data. another id from cookiebot
REACT_APP_ESP_PROXY_DOMAIN # a proxy load balancer domain address. Not needed for now.
REACT_APP_ESP_PROXY_PORT # Port for a proxy load balancer domain address. Not needed for now.
REACT_APP_AUTH0_CLIENT_ID # auth0 client id for making the auth0 login functionality work
REACT_APP_AUTH0_DOMAIN # auth0 domain value

The following are firebase api credentials in order to work with firebase calls.
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REACT_APP_FIREBASE_API_KEY 
REACT_APP_FIREBASE_APP_ID 
REACT_APP_FIREBASE_AUTH_DOMAIN
REACT_APP_FIREBASE_DB_URL 
REACT_APP_FIREBASE_MEASUREMENT_ID
REACT_APP_FIREBASE_MSG_SENDER_ID
REACT_APP_FIREBASE_PROJECT_ID 
REACT_APP_FIREBASE_STORAGE_BUCKET

It would be good to have these documented in our README