Merge pull request #323 from pennam/esp_notes

Add some notes in the README about ESP devices

Author: pennam

README.md

@@ -10,7 +10,7 @@
 ### What?
 The `ArduinoIoTCloud` library is the central element of the firmware enabling certain Arduino boards to connect to the [Arduino IoT Cloud](https://www.arduino.cc/en/IoT/HomePage). The following boards are supported:
 
-* **WiFi**: [`MKR 1000`](https://store.arduino.cc/arduino-mkr1000-wifi), [`MKR WiFi 1010`](https://store.arduino.cc/arduino-mkr-wifi-1010), [`Nano 33 IoT`](https://store.arduino.cc/arduino-nano-33-iot), [`Portenta H7`](https://store.arduino.cc/portenta-h7), `ESP8266`
+* **WiFi**: [`MKR 1000`](https://store.arduino.cc/arduino-mkr1000-wifi), [`MKR WiFi 1010`](https://store.arduino.cc/arduino-mkr-wifi-1010), [`Nano 33 IoT`](https://store.arduino.cc/arduino-nano-33-iot), [`Portenta H7`](https://store.arduino.cc/portenta-h7), [`ESP8266`](https://github.com/esp8266/Arduino/releases/tag/2.5.0), [`ESP32`](https://github.com/espressif/arduino-esp32/releases/tag/1.0.6)
 * **GSM**: [`MKR GSM 1400`](https://store.arduino.cc/arduino-mkr-gsm-1400-1415)
 * **5G**: [`MKR NB 1500`](https://store.arduino.cc/arduino-mkr-nb-1500-1413)
 * **LoRa**: [`MKR WAN 1300/1310`](https://store.arduino.cc/mkr-wan-1310)
@@ -79,3 +79,10 @@ ArduinoCloud.begin(ArduinoIoTPreferredConnection, false).
 ArduinoIoTCloudTCP::handle_SubscribeMqttTopics could not subscribe to /a/t/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/e/i
 ```
 In this case either the device has not been associated with the thing within the Arduino IoT Cloud GUI configuration or there's a typo in the thing id.
+
+#### ESP Boards
+Support for ESP boards is obtained through third-party core with some differences and limitations compared to Arduino boards.
+
+- **Authentication scheme**: Board authentication is done through `DEVICE_LOGIN_NAME` and `DEVICE_KEY`, both values are included in the `thingProperties.h` file.
+- **RTC**: RTC support is not included thus each `ArduinoCould.update()` call will lead to an NTP request introducing delay in your `loop()` function. The scheduler widget will not work correctly if connection is lost after configuration.
+- **Watchdog**: Watchdog support is not included.