Host server hardening/lock-down - add documentation

[breznak]

Use-case:

• as the docker image may be aimed at users looking for a working out-of-the-box solution, we should add instructions how to secure/lock-down the host system, as the security of this service may be compromised by gaining access to the host (which will have a public IP).
• the hardening cannot be implemented in this image, so only provide the instructions.

Some materials: https://www.linode.com/docs/networking/vpn/set-up-a-hardened-openvpn-server

• Please also review the part on "hardening OpenVPN", is that (all?) used here? http://blog.dornea.nu/2015/11/17/openvpn-for-paranoids/ General server: https://www.cyberciti.biz/tips/linux-security.html

[kylemanna]

This exists: https://github.com/kylemanna/docker-openvpn/blob/master/docs/paranoid.md