Add example Donate addon cron command

Add example Donate addon cron command using wget

addons/Donate/settings.php

@@ -383,9 +383,12 @@
 	<br />
 	<code>http://<?php echo $_SERVER['SERVER_NAME']; ?>/addons/Donate/sync.php?key=<?php echo $unique_key; ?></code>
 	<br /><br />
+	<strong>for example</strong> (will run every 20 minutes):<br />
+	<code>wget --spider "http://<?php echo $_SERVER['SERVER_NAME']; ?>/addons/Donate/sync.php?key=<?php echo $unique_key; ?>" >/dev/null 2>&1</code>
+	<br /><br />
 	<strong>Please keep the above URL a secret!</strong>
 	<br /><br />
-	To avoid using the API too often, please leave a reasonable time period between running the cron job.
+	To avoid using the API too often, please leave a reasonable time period between running the cron job, such as 20 minutes.
 
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