Add docs on wiping CPU

Problem

CPU comes with some other firmware that prevents you from downloading the Signet firmware to the device.

Solution

Run this to wipe it:

sudo dfu-util -a 0 -s 0x8000000:unprotect:force -D signet-fw-$(cat firmware-version).dfu

Now the normal flashing command will work.

This ticket is to document this in the README.