Added clarification about local_vars.php.

Shell-plugin.md

@@ -1,6 +1,17 @@
 Runs a given Shell command.
 
-**Note: ** _You must have defined, and set to true, the `ENABLE_SHELL_PLUGIN` flag in vars.php for this plugin to run.
+**Note: ** Because this plugin could potentially be abused, it requires extra steps to enable it:
+
+1. In the root of your PHPCI system, in the same directory where you'll find composer.json and vars.php, look for a file local_vars.php. If it does not exist, create it.
+2. In local_vars.php add this code:
+
+```php
+<?php
+
+define('ENABLE_SHELL_PLUGIN', true);
+```
+
+If `ENABLE_SHELL_PLUGIN` is either false or undefined, the shell plugin won't work.
 
 ### Configuration Options: