Update the README with current requirements, and fix style
Update the README with current requirements, and fix style

The README was still referencing gp-xerces even though it's no
longer required for ORCA, so this was removed. On top of that,
some Markdown formatting fixes, minor wordsmithing, typo fixes
as well as some whitespace cleanup and turned URLs into links.

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