Because if open drivers set your PC on fire, that'd be better than spending some time fixing the blobs /s
So.. Since mhwd (praise it) only supports whatever pacman supports, you are gonna need to add it a custom repo. But opening notepads is annoying, so here you are with a comfy command to do this (only use it once):
sudo sh -c "echo -e '[legacyhw]\nSigLevel = Optional TrustAll\nServer = file://$HOME/packages' >> /etc/pacman.conf"
Now, just download this repository and follow the instructions inside your folder of interest. Please read them, it's not nice to reboot to a black screen.
Only catalyst and nvidia304 are road tested for the moment.