Add an example systemd unit

[dstrelnikov]

I think it might be a good idea to add an example systemd unit to the repo. Here is what works for me.

[Unit]
Description=Car Connectivity
After=network-online.target

[Service]
Type=simple
Restart=always
Environment=PYTHONUNBUFFERED=1
WorkingDirectory=/home/me/.local/venvs/carconnectivity/
ExecStart=/home/me/.local/venvs/carconnectivity/bin/carconnectivity-cli config.json events

[Install]
WantedBy=multi-user.target