Update docs to include basic iptables rules to allow mod_cluster multicast traffic

By default RHEL/Fedora iptables rules blocks multicast traffic. At least on a single JBoss node with Apache and JBoss on the same machine the following iptables rule allows multicast traffic. If this rule isn't in place http://localhost/mod_cluster-manager will not show any nodes as multicast is blocked and Apache<->JBoss won't communicate

/sbin/iptables -I INPUT 5 -p udp -d 224.0.1.0/24 -j ACCEPT
/etc/init.d/iptables save

It may also be helpful to note the iptables rules when running in a cluster also as seen here: http://www.redhat.com/docs/en-US/JBoss_Enterprise_Application_Platform/5.0.1/html/Seam_Reference_Guide/ClusteringAndEJBPassivation.html#clustering.deployment