platform: arduino: mqtt: print where logs get sent to

src/platform/arduino/MQTTTelemetry.cpp

@@ -118,6 +118,8 @@ MQTTTelemetry::handleEventOnline(const InputEvent& evt)
     Log.notice("Connecting to MQTT as %s on %s...", Platform::deviceID(), Device.availabilityTopic.c_str());
     if (m_mqtt.connect(Platform::deviceID(), NULL, NULL, Device.availabilityTopic.c_str(), 0, true, "offline")) {
       Log.notice("Connected to MQTT");
+      String logTopic = m_debugTopic + "/log";
+      Log.info("MQTT logs are available at %s", logTopic.c_str());
       m_needHeartbeat = true;
 
       m_json.clear();