Possible addition to README for use of NAT

[P Cause]

Thanks for ocserv.  I was able to build easily and start and connect but 
network routing wasn't working. After a lot of time and searching I 
found these steps and thought perhaps the README could be updated to add 
the steps.  I found at 
https://www.vultr.com/docs/setup-openconnect-vpn-server-for-cisco-anyconnect-on-ubuntu-14-04-x64 
  What I suggest be added:

Steps to enable NAT

Enable NAT in iptables

iptables -t nat -A POSTROUTING -j MASQUERADE

Enable IPv4 forwarding. Edit the file /etc/sysctl.conf.

net.ipv4.ip_forward=1

Apply the changes

sysctl -p /etc/sysctl.conf