added note re: third-party emailer pluginses

public/templates/admin/settings.tpl

@@ -41,6 +41,9 @@
 	<h3>Email Settings</h3>
 	<div class="alert alert-warning">
 		<div>
+			<p>
+				Please ensure that you have installed a third-party emailer (e.g. PostageApp, Mailgun, Mandrill, SendGrid, etc), otherwise emails will not be sent by NodeBB
+			</p>
 			<p>
 				<strong>Email Address</strong><br />
 				The following email address refers to the email that the recipient will see in the "From" and "Reply To" fields.