Add jumper and resistor for dual-mode floppy emu (20-pin)

[baldengineer]

Floppy Emu supports dual-drives on 20-pin header with a 470 ohm hack.

https://shop.bigmessowires.com/products/dual-525-adapter-for-disk-ii-interface-card

Add resistor and header to enable this support on the 20pin.