[CentOS-docs] Added BackupPC Guide

Scott Dowdle wrote:

> Wow.  Thanks for the BackupPC guide.  I've looked at BackupPC a few times over the last couple of years but haven't gotten around to actually trying to deploy it yet because something always got bumped up in front of as being a more important task... mainly because the modest rsync/rdiff-backup setup was working well enough.  Your guide will enable me to deploy in a more timely and productive manner when the time does come.

Thanks, Scott.