Add documentation to pullup mode

[veillette]

Using the arduino ide, it is possible to set the mode of a pin to INPUT, OUTPUT and INPUT_PULLUP. This last mode allows the client to use the high impedance of the internal pins to measure the voltage across it.

P5bot documentation doesn't not refer to the input_pullup mode but it is possible to use this mode using the standard firmata and pinmode

var pin= b.pin(pinNumber,'DIGITAL', 'PULLUP');