|Finally GPIO success!|
The setup was quite easy I used my breadpi on the breadboard for the first time. The TX on the Pi goes directly to the RX on the Arduino. The TX from the Arduino outputs 5 Volts and that is harmful for the 3.3V Pi so I created a voltage divider with two 10KOhm resistors.
I could not get the automatic reset to work so I resetted the Arduino board by hand, since I do not have a reset button on the board I used a cable from GND touching reset.
I hope to use the Raspberry Pi to collect sensor-readings from my robot.