create a Systemd service file.

This is so the program can run unattended. It is a move away from the
'@reboot' cronjob used previously. Note this will need to be copied into
the appropriate loction of the system if you have just cloned the
repository. Search for Systemd docuementation on the interwebs or in the
repositories README (hopefully it's been added after this commmit).

relay.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Relay service for Return to Ritherdon project by Nicola Ellis
+After=network.service
+
+[Service]
+Type=simple
+ExecStart=bash /home/rtrp/repos/relay/startup.sh
+WorkingDirectory=/home/rtrp/repos/relay
+StandardOutput=inherit
+StandardError=inherit
+Restart=always
+RestartSec=60
+user=rtrp
+
+[Install]
+WantedBy=multi-user.target