standard python and interrupt - update documentation

this library works very well (little modification) in conjunction with python3 on a raspberry, a Adafruit RFM69 breakout board and the interrupt feature (DIO0). In my case, the raspberry work as a bridge between a RFM69-network and LAN.

To use this:
`io.setup(self.dio0_pin, io.IN,pull_up_down=io.PUD_DOWN)`

and

`io.add_event_detect(self.dio0_pin, io.RISING, callback = the_script_to_handle_the data)`

maybe people are interested in this.