Add a link to the MQTT wiki page

If someone is new to mqtt they may not know what this form is asking them, so link to the wiki to provide extra context / examples.

webv4/components/mqtt.xjs

@@ -106,6 +106,10 @@
                                 </div>
                             </div>
                         </form>
+
+                        <div class="alert alert-warning">
+                            If you're not sure what to put here, take a look at the <a href="https://wiki.synchro.net/howto:mqtt#websockets" target="_blank">MQTT Wiki Entry</a>.
+                        </div>
                     </div>
                     <div class="panel-footer">
                         <p>