Update the KB for linux-rdp.md
Update the KB for linux-rdp.md

I have added the sections for RHEL6x, CENTOS6x, and CENTOS7x to provide support for additional operating systems.
robertlesieur/PublicKB,anthonyvetter/PublicKB,DLM1/PublicKB,StephanieWong/PublicKB,ack/PublicKB,jwebs1/PublicKB,robcaldwell/PublicKB,mfar78/PublicKB,jtowns3/PublicKB,jwcolbert/PublicKB,daveshac/PublicKB