Updated readme, and added screenshot

Author: tbowmo

README.md

@@ -3,11 +3,12 @@
 This repo contains the custom `package_*_index.json` files that can be used to add new
 third party boards to the Arduino v1.6.4+ IDE.
 
-Add the following URL to arduino, to enable MySensors boards in the arduino
-IDE
+To add MySensors to Boards Manager list, go to IDE preferences, and at the bottom of the dialog add the following URL into "Additional boards managers URLs:" field and hit OK:
 
 https://raw.githubusercontent.com/mysensors/ArduinoBoards/master/package_mysensors.org_index.json
 
+![Arduino preferences dialog](/screenshot/arduino-preferences-dialog.png?raw=true "Arduino preferences dialog")
+
 ## List of 3rd Party Boards
 
 https://github.com/arduino/Arduino/wiki/Unofficial-list-of-3rd-party-boards-support-urls