[CentOS-docs] Added BackupPC Guide

Ralph Angenendt wrote:
> specific information we put CentOS 3, CentOS 4, CentOS 5 sections into
> documents, so maybe this is something which can/should be done as well
> for arch specific information, too?

Not sure, but perhaps a comment box at the bottom of the page that has 
Arch specific notes ?

Btw, where should I look for editorial notes ? I just looked and failed 
at finding anything. Although that might be the Orval from last night 
still doing the rounds in my head.

- KB