Add information on environment variables to CONTRIBUTING.md

[Mohammed-Shoaib]

Add a section specifying to reach out to @richford when working on roar-firekit since these variables are needed to get the unit tests working with jest.

export ROAR_CI_USER_EMAIL="<SECRET>"
export ROAR_CI_USER_PASSWORD="<SECRET>"
export ROAR_FIREBASE_API_KEY="<SECRET>"
export ROAR_FIREBASE_APP_ID="<SECRET>"
export ROAR_FIREBASE_MEASUREMENT_ID="<SECRET>"
export ROAR_FIREBASE_MESSAGING_SENDER_ID="<SECRET>"
export ROAR_FIREBASE_AUTH_DOMAIN="<SECRET>"
export ROAR_FIREBASE_PROJECT_ID="<SECRET>"
export ROAR_FIREBASE_STORAGE_BUCKET="<SECRET>"