Add "mtu related problems" section to readme

[Lazhur]

As mtu related dropping of packets seems to be the biggest problem using batman-adv it is maybe a good idea to add a small section about it to the readme.

It should have following parts:

• Why mtu is lower for bat0
• Where packets are dropped in a typical scenario:

  client            brigded node1      bridged node2     server
  [eth0 (1500)] <-> [eth0 (1500)       [eth0 (1500)  <-> [eth0 (1500)]
                     bat0 (1476)]       bat0 (1476)]
                      ||                 ||
                    [wlan0 (1500)] <-> [wlan0 (1500)]
  

• How it can be tested (ping -M do -s 1472 $IP)
• Effects on connections (hangs, connection loss, extreme slow tests with iperf due to TCP window scaling going wild, ...)
• What can be done against it (wlan0: mtu 1524; br0, eth0, bat0: mtu 1500)
• Effects of Simons PMTU bridge patch