readme: moved user mapping sectio before `Shell Access`
readme: moved user mapping sectio before `Shell Access`

timclassic/docker-postgresql,logandg/docker-postgis,zires/docker-postgresql,markrowsoft/docker-postgres-bdr,boky8/docker-postgresql,crobays/docker-postgresql,arkkanoid/docker-postgresql,sameersbn/docker-postgresql,bokysan/docker-postgresql,whatever4711/rpi-postgresql,mental-docker/postgresql