Added specifics for systemd services when using a keybase bridge

_Footer.md

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+## Headless server install + systemd
+
+If you run matterbridge as a service in systemd and want to use a bridge with keybase, be sure to run the service as the default user via *user.slice*, since keybase also runs its services in *user.slice*.
+
+### Example service
+
+Create a matterbridge.service file in 
+
+`/usr/lib/systemd/user/`
+
+->
+
+```
+[Unit]
+Description=matterbridge
+After=network.target
+
+[Service]
+ExecStart=/path-to-your-executable/matterbridge
+Restart=on-failure
+RestartSec=10s
+
+[Install]
+WantedBy=multi-user.target
+```
+
+*Start service* - `systemctl --user start matterbridge.service`
+*Stop service* - `systemctl --user stop matterbridge.service`
+*Check Status* - `systemctl --user status matterbridge.service`
+
+*Start at System Startup* - `systemctl --user enable matterbridge.service`
+**If you are using an SSH session, user services might shut down, use this to prevent:**  `loginctl enable-linger`