README:  add URLs for openldap installation
README:  add URLs for openldap installation

Reported-by: Michael Paquier

Discussion: https://postgr.es/m/20180521013425.GA4476@paquier.xyz

Backpatch-through: head

xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb