Add warning about hub scale units that support ADU

examples/Azure_IoT_Adu_ESP32/readme.md

@@ -52,6 +52,7 @@ _The following was run on Windows 11 and Ubuntu Desktop 20.04 environments, with
 
 - Have an [Azure account](https://azure.microsoft.com/) created.
 - Have an [Azure IoT Hub](https://docs.microsoft.com/azure/iot-hub/iot-hub-create-through-portal) created.
+  > Important: To use Azure Device Update you must have an Azure IoT Hub with [scale unit](https://learn.microsoft.com/en-us/azure/iot-hub/iot-hub-scaling) S1, S2, S3 or a free/standard hub. This sample will not work with a Basic Azure IoT Hub scale unit. 
 - Have an [Azure Device Update](https://docs.microsoft.com/azure/iot-hub-device-update/create-device-update-account?tabs=portal) instance created and linked to your Azure IoT Hub.
 - Have a logical device created in your Azure IoT Hub: using authentication type "Symmetric Key" or "X.509 self-signed".   
   - **Symmetric Key**: follow [this guidance](https://docs.microsoft.com/azure/iot-hub/iot-hub-create-through-portal#register-a-new-device-in-the-iot-hub) to create a device.In this case, the device keys are used to automatically generate a SAS token for authentication.