Added note about password and privileges for OSM DB user

docker/postgres/openstreetmap-postgres-init.sh

@@ -2,6 +2,7 @@
 set -ex
 
 # Create 'openstreetmap' user
+# Password and superuser privilege are needed to successfully run test suite
 psql -v ON_ERROR_STOP=1 -U "$POSTGRES_USER" <<-EOSQL
     CREATE USER openstreetmap SUPERUSER PASSWORD 'openstreetmap';
     GRANT ALL PRIVILEGES ON DATABASE openstreetmap TO openstreetmap;