Add a note to /etc/default/docker noting that it does not apply to systemd

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

contrib/init/sysvinit-debian/docker.default

@@ -1,5 +1,12 @@
 # Docker Upstart and SysVinit configuration file
 
+#
+# THIS FILE DOES NOT APPLY TO SYSTEMD
+#
+#   Please see the documentation for "systemd drop-ins":
+#   https://docs.docker.com/engine/articles/systemd/
+#
+
 # Customize location of Docker binary (especially for development testing).
 #DOCKER="/usr/local/bin/docker"