Update docs

- Add links to example steps for connecting to IKEv2 VPN
  from Linux using the command line.
  Thanks to @Alphalynxjet, @neothematrix and others for
  sharing these example steps.

docs/ikev2-howto-zh.md

@@ -455,6 +455,8 @@ sudo chmod 600 ca.cer client.cer client.key
 1. 单击 **Add** 保存 VPN 连接信息。
 1. 启用 **VPN** 连接。
 
+另外，你也可以使用命令行连接。示例步骤请参见 [#1399](https://github.com/hwdsl2/setup-ipsec-vpn/issues/1399) 和 [#1007](https://github.com/hwdsl2/setup-ipsec-vpn/issues/1007)。
+
 连接成功后，你可以到 [这里](https://www.ipchicken.com) 检测你的 IP 地址，应该显示为`你的 VPN 服务器 IP`。
 
 如果在连接过程中遇到错误，请参见 [故障排除](#ikev2-故障排除)。

docs/ikev2-howto.md

@@ -457,6 +457,8 @@ You can then set up and enable the VPN connection:
 1. Click **Add** to save the VPN connection information.
 1. Turn the **VPN** switch ON.
 
+Alternatively, you may connect using the command line. See [#1399](https://github.com/hwdsl2/setup-ipsec-vpn/issues/1399) and [#1007](https://github.com/hwdsl2/setup-ipsec-vpn/issues/1007) for example steps.
+
 Once connected, you can verify that your traffic is being routed properly by [looking up your IP address on Google](https://www.google.com/search?q=my+ip). It should say "Your public IP address is `Your VPN Server IP`".
 
 If you get an error when trying to connect, see [Troubleshooting](#ikev2-troubleshooting).