Update samples readme with link to Azure Device Update sample (#2399)

* Update samples readme with link to Azure Device Update sample

* Clarify that new ADU sample is over Arduino IDE

sdk/docs/iot/README.md

@@ -53,6 +53,8 @@ For more detailed, step-by-step guides to setup and run the IoT Hub certificate
 
 - Espressif ESP32: [How to Setup and Run Azure SDK for Embedded C IoT Hub Client on ESP32](https://github.com/Azure/azure-sdk-for-c/blob/main/sdk/samples/iot/aziot_esp32/readme.md)
 
+- Azure Device Update with Espressif ESP32 on Arduino IDE: [How to Setup and Run Azure SDK for Embedded C ADU on Espressif ESP32](https://github.com/Azure/azure-sdk-for-c-arduino/blob/main/examples/Azure_IoT_Adu_ESP32/readme.md)
+
 - VxWorks: [How to Setup and Run Azure SDK for Embedded C IoT Hub Client and Provisioning Client Samples on VxWorks](https://github.com/Azure/azure-sdk-for-c/blob/main/sdk/samples/iot/docs/how_to_iot_hub_samples_vxworks.md)
 
 **Important Note on Linux and Windows Samples**: While Windows and Linux devices are not likely to be considered constrained, these samples enable developers to test the Azure SDK for Embedded C libraries, debug, and step through the code, even without a real device. We understand not everyone will have a real device to test and that sometimes these devices won't have debugging capabilities.