Updating documentation around existing headless user addition feature (#488)

This closes #422

README.md

@@ -70,6 +70,19 @@ Other variables can be set depending on your choice (encryption, compression). Y
 
 Password-protected clients are not supported by the headless installation method since user input is expected by Easy-RSA.
 
+### Headless User Addition
+
+It's also possible to automate the addition of a new user. Here, the key is to provide the (string) value of the `MENU_OPTION` variable along with the remaining mandatory variables before invoking the script.
+
+The following Bash script adds a new user `foo` to an existing OpenVPN configuration
+```bash
+#!/bin/bash
+export MENU_OPTION="1"
+export CLIENT="foo"
+export PASS="1"
+./openvpn-install.sh
+```
+
 ## Features
 
 - Installs and configures a ready-to-use OpenVPN server