conf: add an example snippet for how to assign options

Users still like to copy the whole file, potentially messing things up.
Let's put a warning into the file directly that this is less than ideal.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

conf/40-libinput.conf

@@ -1,4 +1,21 @@
 # Match on all types of devices but joysticks
+#
+# If you want to configure your devices, do not copy this file.
+# Instead, use a config snippet that contains something like this:
+#
+# Section "InputClass"
+#   Identifier "something or other"
+#   MatchDriver "libinput"
+#
+#   MatchIsTouchpad "on"
+#   ... other Match directives ...
+#   Option "someoption" "value"
+# EndSection
+#
+# This applies the option any libinput device also matched by the other
+# directives. See the xorg.conf(5) man page for more info on
+# matching devices.
+
 Section "InputClass"
         Identifier "libinput pointer catchall"
         MatchIsPointer "on"