Update docs

- Ref: #1209

README-zh.md

@@ -85,6 +85,8 @@ https://gitlab.com/hwdsl2/setup-ipsec-vpn/-/raw/master/vpnsetup.sh
 
 [**» 我想建立并使用自己的 VPN，但是没有可用的服务器**](https://blog.ls20.com/ipsec-l2tp-vpn-auto-setup-for-ubuntu-12-04-on-amazon-ec2/#gettingavps)
 
+对于有外部防火墙的服务器（比如 [EC2](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-security-groups.html)/[GCE](https://cloud.google.com/vpc/docs/firewalls)），请为 VPN 打开 UDP 端口 500 和 4500。
+
 另外，你也可以使用预构建的 [Docker 镜像](https://github.com/hwdsl2/docker-ipsec-vpn-server/blob/master/README-zh.md)。高级用户可以在 [Raspberry Pi](https://www.raspberrypi.org) 上安装。[[1]](https://elasticbyte.net/posts/setting-up-a-native-cisco-ipsec-vpn-server-using-a-raspberry-pi/) [[2]](https://www.stewright.me/2018/07/create-a-raspberry-pi-vpn-server-using-l2tpipsec/)
 
 :warning: **不要** 在你的 PC 或者 Mac 上运行这些脚本！它们只能用在服务器上！

README.md

@@ -85,6 +85,8 @@ This also includes Linux VMs in public clouds, such as [DigitalOcean](https://bl
 
 [**» I want to run my own VPN but don't have a server for that**](https://blog.ls20.com/ipsec-l2tp-vpn-auto-setup-for-ubuntu-12-04-on-amazon-ec2/#gettingavps)
 
+For servers with an external firewall (e.g. [EC2](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-security-groups.html)/[GCE](https://cloud.google.com/vpc/docs/firewalls)), open UDP ports 500 and 4500 for the VPN.
+
 A pre-built [Docker image](https://github.com/hwdsl2/docker-ipsec-vpn-server) is also available. Advanced users can install on a [Raspberry Pi](https://www.raspberrypi.org). [[1]](https://elasticbyte.net/posts/setting-up-a-native-cisco-ipsec-vpn-server-using-a-raspberry-pi/) [[2]](https://www.stewright.me/2018/07/create-a-raspberry-pi-vpn-server-using-l2tpipsec/)
 
 :warning: **DO NOT** run these scripts on your PC or Mac! They should only be used on a server!