Updated REAMDE.md

Signed-off-by: David Jander <djander@gmail.com>
pid.py: Implemented basic safety functions

Detect a temperature sensor- or measurement circuit defect and turn off
the corresponding heater. This is by no means a safety function that
should be trusted blindly, but it can reduce the chances of fire a bit.

Signed-off-by: David Jander <djander@gmail.com>