easy intro to HA and fencing link put into wiki

[m.ardito]

for anybody (like me) needing an easy intro to HA and fencing, i've found a nice document, i've put a link on the wiki here

http://pve.proxmox.com/wiki/Fencing#Introduction

well, just read the begin of the linked page, as the rest is referring to other software, not pve

hth,
Marco