Add instructions on how to backup the PostgreSQL

It might be good to have a little "if you're using the docker compose files, this is how you can export the db to back it up" section and then restore it again

currently i'm doing

docker run --rm \
    --volumes-from panoramax2-mapcomplete-db-1 \
    -v $(pwd)/backups:/backup \
    busybox tar cvf /backup/postgres_data_backup_$(date +%F).tar /var/lib/postgresql/data/

Is that ok?

(what happens if you restore a backup from a 'latest' install to a fresh 'latest' install from a newer version. And db migrations happened between those. will that work?)