add note about ModemManager

[nmschulte]

This is interesting. FWIW, I've not had a problem with ModemManager on my Debian host. You can probably also tell ModemManager to stop what it's doing, avoiding a system-wide/-level configuration (via e.g. mmcli or w/e GUI is preferred).

[chuckwagoncomputing]

I recall this happening to me once with an Arduino, and I just killed the service with systemctl, but it hasn't happened with rusEFI for me.