README documentation about helper service ports is unclear

The README.md indicates you can change values such as POSTGRES_PORT, MONGO_PORT, etc. in your .env file. While this partially works due to the environment part of the cdcs service definition in docker-compose.yml:

    environment:
      - DJANGO_SETTINGS_MODULE=${PROJECT_NAME}.${SETTINGS}
      - DJANGO_SECRET_KEY=${DJANGO_SECRET_KEY}
      - SERVER_URI=${SERVER_URI}
      - ALLOWED_HOSTS=${ALLOWED_HOSTS}
      - SERVER_NAME=${SERVER_NAME:-}
      - MONGO_HOST=${PROJECT_NAME}_cdcs_mongo
      - MONGO_PORT=${MONGO_PORT:-27017}
      - MONGO_DB=${MONGO_DB}
      - MONGO_USER=${MONGO_USER}
      - MONGO_PASS=${MONGO_PASS}
      - POSTGRES_HOST=${PROJECT_NAME}_cdcs_postgres
      - POSTGRES_PORT=${POSTGRES_PORT:-5432}
      - POSTGRES_DB=${POSTGRES_DB}
      - POSTGRES_USER=${POSTGRES_USER}
      - POSTGRES_PASS=${POSTGRES_PASS}
      - REDIS_HOST=${PROJECT_NAME}_cdcs_redis
      - REDIS_PORT=${REDIS_PORT:-6379}
      - REDIS_PASS=${REDIS_PASS}
      - MONITORING_SERVER_URI=${MONITORING_SERVER_URI:-}
      - ELASTICSEARCH_HOST=${ELASTICSEARCH_HOST:-}
      - ELASTICSEARCH_PORT=${ELASTICSEARCH_PORT:-}

It doesn't actually work when you try to bring the deployment up, because the individual services (PostgreSQL, Mongo, and Redis) haven't changed what port they're running on internally (and they don't expose ports onto the host). Since docker-compose uses internal networking for all of these services, changing the ports isn't actually necessary if you want to run multiple CDCS instances on the same machine, since the ports are local to the individual services, and won't conflict on the host (since they're not being exposed). The only ones that need to be changed are the nginx ones.

I would suggest adding a note to the description field for these values in the README.md field that changing them will break things (and it's not necessary, as described above).

usnistgov / cdcs-docker

README documentation about helper service ports is unclear #4