Adding additional content on network setup

Signed-off-by: Lucas A. Wilson <luke.wilson@dell.com>

docs/PREINSTALL.md

@@ -19,4 +19,5 @@ Omnia can configure systems which use Ethernet- or Infiniband-based fabric to co
 
 ## Network Setup
 Omnia assumes that servers are already connected to the network and have access to the internet. Possible network configurations include:
-* 
+* A flat topology where all nodes are connected to a switch which includes an uplink to the internet. This requires multiple externally-facing IP addresses
+* A hierarchical topology where compute nodes are connected to a common switch, but the master node contains a second network connection which is connected to the internet. All outbound/inbound traffic would be routed through the master node. This requires setting up firewall rules for IP masquerade, see (https://www.server-world.info/en/note?os=CentOS_7&p=firewalld&f=2) for an example.