Update the example systemd config to use a virtualenv (#4273)

If you're installing as a system package, the system package should have set up
the systemd config, so it's more useful to give an example of running in a
virtualenv here.

changelog.d/4273.misc

+Update the example systemd config to use a virtualenv

contrib/systemd/synapse.service → contrib/systemd/matrix-synapse.service

-# This assumes that Synapse has been installed as a system package
-# (e.g. https://www.archlinux.org/packages/community/any/matrix-synapse/ for ArchLinux)
-# rather than in a user home directory or similar under virtualenv.
-
+# Example systemd configuration file for synapse. Copy into
+#    /etc/systemd/system/, update the paths if necessary, then:
+#
+#    systemctl enable matrix-synapse
+#    systemctl start matrix-synapse
+#
+# This assumes that Synapse has been installed in a virtualenv in
+# /opt/synapse/env.
+#
 # **NOTE:** This is an example service file that may change in the future. If you
 # wish to use this please copy rather than symlink it.
 
@@ -10,12 +15,16 @@ Description=Synapse Matrix homeserver
 
 [Service]
 Type=simple
+Restart=on-abort
+
 User=synapse
-Group=synapse
-WorkingDirectory=/var/lib/synapse
-ExecStart=/usr/bin/python2.7 -m synapse.app.homeserver --config-path=/etc/synapse/homeserver.yaml
-ExecStop=/usr/bin/synctl stop /etc/synapse/homeserver.yaml
-# EnvironmentFile=-/etc/sysconfig/synapse  # Can be used to e.g. set SYNAPSE_CACHE_FACTOR 
+Group=nogroup
+
+WorkingDirectory=/opt/synapse
+ExecStart=/opt/synapse/env/bin/python -m synapse.app.homeserver --config-path=/opt/synapse/homeserver.yaml
+
+# adjust the cache factor if necessary
+# Environment=SYNAPSE_CACHE_FACTOR=2.0
 
 [Install]
 WantedBy=multi-user.target