Add the esp8266 boards manager link to readme

[claesgill]

So we don't have to type it out from the video every time 😅

Section should say smtl:
Go to: Settings (Windows Preferences)
In additional boards manager URLs add: http://arduino.esp8266.com/stable/package_esp8266com_index.json
Go to the Boards manager and install esp8622 by ESP8266 Community.