examples/logrotate.conf: update example for systemd

...And add placeholders for other systems

examples/logrotate.conf

@@ -3,6 +3,9 @@
 #
 # See the logrotate(8) manpage for more information:
 #    http://linux.die.net/man/8/logrotate
+#
+# public logrotate-related discussion in our archives:
+#    https://bogomips.org/unicorn-public/?q=logrotate
 
 # Modify the following glob to match the logfiles your app writes to:
 /var/log/unicorn_app/*.log {
@@ -22,7 +25,19 @@
 	# config.  Unicorn supports the USR1 signal and we send it
 	# as our "lastaction" action:
 	lastaction
-		# assuming your pid file is in /var/run/unicorn_app/pid
+		# For systemd users, assuming you use two services
+		# (as recommended) to allow zero-downtime upgrades.
+		# Only one service needs to be started, but signaling
+		# both here is harmless as long as they're both enabled
+		systemctl kill -s SIGUSR1 unicorn@1.service
+		systemctl kill -s SIGUSR1 unicorn@2.service
+
+		# Examples for other process management systems appreciated
+		# Mail us at unicorn-public@bogomips.org
+		# (see above for archives)
+
+		# If you use a pid file and assuming your pid file
+		# is in /var/run/unicorn_app/pid
 		pid=/var/run/unicorn_app/pid
 		test -s $pid && kill -USR1 "$(cat $pid)"
 	endscript