Added instructions to install udev rules for immutable systems

Since /usr/lib/udev/rules.d/ is read-only on immutable systems, the user should instead use /etc/udev/rules.d/. This could also possibly be implemented in the udev rules install script.

I've also fixed the link to the udev rules not working. However I'm not sure if it's the one we want since it's tied to the 0.9 release and not to gitlab.