add the 'temperature' file to set/get temperature compensation value

The external program shuold set this just before issuing the update command.

The internal temperature sensor could be wired to a suitable ADC to provide this
as there is no ADC channel on the basic board.

Signed-off-by: Christopher Hall <christopherhall.hsw@gmail.com>
Update readme with latest driver changes

Signed-off-by: Christopher Hall <christopherhall.hsw@gmail.com>