add examples

contrib/helper.sh

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+#!/bin/sh -f
+#
+# this helper required for CONFIG_UEVENT_HELPER aka /proc/sys/kernel/hotplug
+# for mdev you can use simple one-liner, refer to mdev.conf for more info
+#
+# usage:
+# echo /full/path/to/helper.sh > /proc/sys/kernel/hotplug
+# echo "/full/path/to/helper.sh UDEV_MONITOR_DIR" > /proc/sys/kernel/hotplug
+#
+
+# NOTE: writing variables to file (e.g PWD or PATH)
+# that are not related to uevent properties is harmless
+env > "${1:-/tmp/.libudev-zero}/uevent.$$"

contrib/mdev.conf

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+#
+# example basic mdev config representing libudev-zero usage 
+# uncomment needed rules
+#
+# NOTE: you must change "/tmp/.libudev-zero" if you use non-default UDEV_MONITOR_DIR
+# NOTE: mdev can only handle ADD and/or REMOVE events
+
+# will handle all uevents
+#-.* root:root 660 *env > /tmp/.libudev-zero/uevent.$$
+
+# will handle only drm and input uevents (recommended)
+#SUBSYSTEM=drm;.*   root:video 660 *env > /tmp/.libudev-zero/uevent.$$
+#SUBSYSTEM=input;.* root:input 660 *env > /tmp/.libudev-zero/uevent.$$