Update the example systemd config to use a virtualenv (matrix-org#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

@@ -0,0 +1 @@
+Update the example systemd config to use a virtualenv

contrib/systemd/matrix-synapse.service

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+# 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.
+
+[Unit]
+Description=Synapse Matrix homeserver
+
+[Service]
+Type=simple
+Restart=on-abort
+
+User=synapse
+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
+