Add Arduino core dep.

Adding Arduino core dependency

Author: bram2202

README.md

@@ -11,6 +11,7 @@ The code should work on DSRM v2.2 and higher, only tested on V4.2.
 ## Requirements 
 * ESP8266 (Wemos/LOLIN D1 mini/ESP01/NodeMCU)
 * Basic soldering and wiring skills
+* Arduino core for ESP8266 WiFi chip [[link]](https://github.com/esp8266/Arduino)
 * (For Wemos d1 mini) CH340G driver [[link]](https://wiki.wemos.cc/downloads)
 * Arduino IDE
 * Hardware package for arduino [[LINK]](https://github.com/esp8266/Arduino)
@@ -120,4 +121,4 @@ Connecting to the DSMR witn a RJ11 in Port 1 (P1), found on most smart meters.
 - If the level shifter inverter is connected, it's impossible to flash the firmware.<br>
 Pin RX is used, disconnect the pin to flash new firmware.
 - Some DSMR cannot deliver enough power to run the Wemos stably.<br> 
-Connect a 5V usb supply to fix this.
+Connect a 5V usb supply to fix this.