Add commented reference to official arduino board package_index.json

Author: ianfixes

misc/default.yml

@@ -4,6 +4,7 @@
 
 packages:
   # arduino:xxx are builtin, we don't need to include them here
+  # but if we did, it would be url: https://downloads.arduino.cc/packages/package_index.json
   esp8266:esp8266:
     url: http://arduino.esp8266.com/stable/package_esp8266com_index.json
   adafruit:avr: