ci: document arduino pins

ci/hardware_tests/tpmb/README.md

@@ -11,6 +11,11 @@ To build this simple bot, we are using:
 * Arduino UNO R3 (or compatible)
 * 2x Tower Pro Micro Servo 9g SG9
 
+Servos should be plugged in:
+- Both cables with green endings into slots 7 and 8.
+- Red one into 5V power.
+- Brown into ground (GND).
+
 ## Software
 
 * ```arduino/``` includes the code for the hardware