Update Readme - Deprecate Sample (#27)

This sample is not intended for customer use anymore, since we have new ones for ESP32.
It should be kept here for reference only. 
We're also removing the YAML reference so this sample won't be indexed by Azure Samples engine anymore.

Author: wduraes

README.md

@@ -1,3 +1,14 @@
+> ### Stop! Before you proceed:
+>
+> _This Get Started Guide is an **older version** and it is neither maintained nor supported anymore._
+>
+> _It is kept here for **reference only** and should not be used for any new development._
+>
+> _If you’re looking for a Get Started guide to the Espressif ESP32 there are two alternatives available:_
+>
+> 1. _[Azure IoT SDK for C](https://github.com/Azure/azure-sdk-for-c-arduino/blob/main/examples/Azure_IoT_Hub_ESP32/readme.md) which uses a bare metal approach (no RTOS) and support for Arduino IDE._
+> 2. _[Azure IoT middleware for FreeRTOS](https://github.com/Azure-Samples/iot-middleware-freertos-samples) which uses ESP-IDF._
+<br>
 
 # ESP32_AzureIoT - Azure IoT Hub library for esp32 devices in Arduino