Merge pull request #187115 from ebertrams/properties-to-use-routing

add properties to use routing

Author: PRMerger3

articles/iot-hub/iot-hub-mqtt-support.md

@@ -294,6 +294,9 @@ The following is a list of IoT Hub implementation-specific behaviors:
 
 * IoT Hub only supports one active MQTT connection per device. Any new MQTT connection on behalf of the same device ID causes IoT Hub to drop the existing connection and **400027 ConnectionForcefullyClosedOnNewConnection** will be logged into IoT Hub Logs
 
+* To route messages based on message body, you must first add property 'contentType' (`ct`) to the end of the MQTT topic and set its value to be `application/json;charset=utf-8`. An example is shown below. To learn more about routing messages either based on message properties or message body, please see the [IoT Hub message routing query syntax documentation](iot-hub-devguide-routing-query-syntax.md).
+
+    ```devices/{device_id}/messages/events/$.ct=application%2Fjson%3Bcharset%3Dutf-8```
 
 For more information, see [Messaging developer's guide](iot-hub-devguide-messaging.md).