Document: cable setup

[ismith]

An rnet cable needs to be wired in to the PiCAN 2 hat. Better docs may be needed:

• pinout (the pinout in upstream doc uses numbers, but doesn't show which orientation of cable that maps to; also, if you're cutting into the middle of the cable, colors might be more immediately helpful than knowing which pin at the plug end goes to which terminal)
• pics of assembly (visual learners! could even be a video)
• diagram or description - we found that CAN_H and CAN_L were thin enough to wire in to the PiCAN 2's screw terminals, but needed a thinner bit of wire to connect RNet GND to PiCAN 2 GND)

[ismith]

@prileo if we have another rnet cable, we could bring it to the hackathon, have you build it there and another person (me? someone else?) video + photo the whole thing