networking.md: Networking between containers works without ip_forward

My test setup here starts docker with --ip-forward=false --iptables=false. I start two containers on the default docker0 bridge. I have no problems pinging container#1 from container#2, with /proc/sys/net/ipv4/ip_forward set to 0. Which is as expected, since you add all containers to the same bridge by default.

ip_forward is only needed for talking to ip addresses not on the bridge subnet, such as the wider world via nat.