Minor snag, $prefix prefixed to udev rules means /usr/lib/ not /lib

We may need to drop the $prefix to the udev rules from configure.ac

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

README.md

     ./configure --with-udev && make
     sudo make install
 
-This installs the file `90-backlight.rules` into `/lib/udev/rules.d/`.
+This installs the `90-backlight.rules` into `/usr/lib/udev/rules.d/`.
 If your udev rules are located elsewhere, use `--with-udev=PATH`.
 
 Note, in this mode `light` runs unpriviliged, so the `/etc/light`