gpperfmon: Document installing a fork of sigar on CentOS 7
gpperfmon: Document installing a fork of sigar on CentOS 7

https://github.com/hyperic/sigar is not under active development.

https://github.com/boundary/sigar is a fork that has been somewhat updated. In particular, it has a fix to allow it to compile with GCC 5 and 6.
50wu/gpdb,edespino/gpdb,Chibin/gpdb,ashwinstar/gpdb,50wu/gpdb,edespino/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,edespino/gpdb,Chibin/gpdb,lisakowen/gpdb,edespino/gpdb,adam8157/gpdb,xinzweb/gpdb,Chibin/gpdb,jmcatamney/gpdb,lisakowen/gpdb,edespino/gpdb,lisakowen/gpdb,Chibin/gpdb,50wu/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,Chibin/gpdb,xinzweb/gpdb,jmcatamney/gpdb,edespino/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,Chibin/gpdb,Chibin/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,Chibin/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,edespino/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb,edespino/gpdb,lisakowen/gpdb,edespino/gpdb,xinzweb/gpdb,ashwinstar/gpdb,edespino/gpdb,Chibin/gpdb,Chibin/gpdb