Add .env.sample files to corresponding directories

[sadikkuzu]

It would be very nice to have .env.sample files in backend and frontend folders.

./backend/backend/.env.sample
./backend/frontend/.env.sample

---

./backend/backend/.env.sample

SECRET_KEY=django-insecure-kthw4rq
DB_NAME=testdb
DB_USER=postgres
DB_PASSWORD=AwesomePassword
DB_HOST=db
# DB_HOST=localhost
DB_PORT=5432
USER=email_user
PASSWORD=email_password
POSTGRES_USER=postgres
POSTGRES_PASSWORD=AwesomePassword
POSTGRES_DB=testdb
BACKEND_HOST_IP=localhost
DJANGO_LOG_LEVEL=WARNING  # DEBUG, INFO, WARNING, ERROR, CRITICAL

./backend/frontend/.env.sample

REACT_APP_GOOGLE_MAPS_API_KEY=AIzaSg-***************
REACT_APP_BACKEND_HOST_NAME=localhost