Add link to the core release used for ESP8266 boards

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`, [`ESP32`](https://github.com/espressif/arduino-esp32/releases/tag/1.0.6)
+* **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)