Update instructions on port change with systemd.

As suggested by Przemek Sobstel on github.

doc/sphinx/tutorial/putting_varnish_on_port_80.rst

@@ -9,10 +9,10 @@ First we stop varnish: ``service varnish stop``
 
 Now we need to edit the configuration file that starts Varnish.
 
-Debian/Ubuntu
-~~~~~~~~~~~~~
+Debian/Ubuntu (legacy)
+~~~~~~~~~~~~~~~~~~~~~~
 
-On Debian/Ubuntu this is `/etc/default/varnish`. In the file you'll find
+On older Debian/Ubuntu this is `/etc/default/varnish`. In the file you'll find
 some text that looks like this::
 
   DAEMON_OPTS="-a :6081 \
@@ -29,6 +29,26 @@ Change it to::
                -S /etc/varnish/secret \
                -s malloc,256m"
 
+Debian (v8+) / Ubuntu (v15.04+)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+On more recent Debian and Ubuntu systems this is configured in the systemd
+service file.
+
+Applying changes to the default service is best done by creating a new file
+`/etc/systemd/system/varnish.service.d/customexec.conf`::
+
+  [Service]
+  ExecStart=
+  ExecStart=/usr/sbin/varnishd -a :80 -T localhost:6082 -f /etc/varnish/default.vcl -S /etc/varnish/secret -s malloc,256m
+
+This will override the ExecStart part of the default configuration shipped
+with Varnish Cache.
+
+Run ``systemctl daemon-reload`` to make sure systemd picks up the new
+configuration before restarting Varnish.
+
+
 Red Hat Enterprise Linux / CentOS
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -42,4 +62,3 @@ Restarting Varnish again
 Once the change is done, restart Varnish: ``service varnish start``.
 
 Now everyone accessing your site will be accessing through Varnish.
-