Add INSTALL_ARCH.md covering installation on Arch-based distributions,
validated on a Lukton DQ-211 (AIC8800D80, Pandora 88M80 clone) under
CachyOS with kernels 7.2.0-1-cachyos and 6.18.42-1-cachyos-lts.
The notes cover the AUR/DKMS path, the manual build path, and two gotchas
that are not documented for these distros:
- the AUR package ships aic.rules but not /etc/usb_modeswitch.d/1111:1111,
so the mode switch silently does nothing without it;
- the DKMS modules live under updates/dkms/ and need depmod -a before
modprobe aic8800_fdrv can find them.
Also document that aic_load_fw already carries the usb:vA69Cp8D80d* alias,
so no modules-load.d entry is required, plus NetworkManager route-metric
notes for preferring the dongle over the onboard radio.
README: link the new notes from the Installation Steps section.
Co-authored-by: CommandCodeBot <noreply@commandcode.ai>
