docs(gateway): add deployment checklist, systemd service and update README

gateway/gateway.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=BLE to MQTT Gateway
+After=network.target bluetooth.target
+Wants=bluetooth.target
+
+[Service]
+Type=simple
+User=pi
+WorkingDirectory=/home/pi/Plein-de-eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-project/gateway
+EnvironmentFile=/home/pi/secrets/mqtt.env
+ExecStart=/home/pi/Plein-de-eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-project/gateway/venv/bin/python gateway.py
+Restart=on-failure
+RestartSec=10
+
+[Install]
+WantedBy=multi-user.target