Use addresses reserved for documentation in interfaces example file.

Mention two setups:

 * routed setup
    - external router
    - nodes without IP inside routed network
    - proxy ARP

 * bridged setup
    - extra common interfaces on all nodes
    - one common bridge on separate interface
    - node with internet access is the gateway and does NAT

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>