readme: updated contributing section
readme: updated contributing section

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