clarify SWD instructions

[securelyfitz]

• needed to build openocd to get swd working (https://hackaday.io/project/164346-andxor-dc27-badge/log/166464-swd-all-the-things)
• newer openocd has different cfg file requirements (below)
• need to documen this somehow...

adapter driver ftdi
transport select swd
ftdi_vid_pid 0x0403 0x6010
ftdi_channel 1
adapter speed 2000
ftdi_layout_init 0x0018 0x05fb
ftdi_layout_signal SWD_EN -data 0
ftdi_layout_signal nSRST -ndata 0x0020 -noe 0x0040

[securelyfitz]

Fixed in 1275221