Update README.md for building GPDB on centos
Update README.md for building GPDB on centos

Add required steps before building GPDB on centos.
Fixes issue #2715, #2995 and #3163.

[ci skip]

yuanzhao/gpdb,Chibin/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,janebeckman/gpdb,ashwinstar/gpdb,lisakowen/gpdb,edespino/gpdb,adam8157/gpdb,edespino/gpdb,yuanzhao/gpdb,Chibin/gpdb,janebeckman/gpdb,adam8157/gpdb,greenplum-db/gpdb,edespino/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,janebeckman/gpdb,lisakowen/gpdb,edespino/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,janebeckman/gpdb,edespino/gpdb,yuanzhao/gpdb,Chibin/gpdb,yuanzhao/gpdb,Chibin/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,edespino/gpdb,lisakowen/gpdb,janebeckman/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,janebeckman/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,Chibin/gpdb,jmcatamney/gpdb,Chibin/gpdb,edespino/gpdb,jmcatamney/gpdb,Chibin/gpdb,adam8157/gpdb,edespino/gpdb,ashwinstar/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,janebeckman/gpdb,xinzweb/gpdb,edespino/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,Chibin/gpdb,edespino/gpdb,greenplum-db/gpdb,adam8157/gpdb,jmcatamney/gpdb,Chibin/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,yuanzhao/gpdb,Chibin/gpdb,janebeckman/gpdb,50wu/gpdb,50wu/gpdb,ashwinstar/gpdb,50wu/gpdb,janebeckman/gpdb,adam8157/gpdb,ashwinstar/gpdb,lisakowen/gpdb,xinzweb/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,janebeckman/gpdb,50wu/gpdb,ashwinstar/gpdb,yuanzhao/gpdb