add systemd unit file, describe install in readme.

README.md

 Simple MQTT bridge in the form of a Telegram Bot.
 And also my first attempt at doing something with Go.
 
-## Getting Started
+## Configuration
 
 First register a new Bot with the Telegram, as [described in the docs](https://core.telegram.org/bots#6-botfather), to get your API key.
 
-Then simply run lights-telegram once manually.
+Then simply run lights-telegram once manually using `go run .`.
 This will create a `config.yaml` file in the same directory.
 Your API key goes in there, as well as the MQTT credentials.
 
 As an admin use `/register` to add new MQTT topics and their possible values to the menu.
 Finally run `/commandlist` on the bot to get a nicely formatted command list which you can then give to the `/setcommands` command of BotFather.
 
+## Installation
+
+Run `go build` to produce an executable in the project directory.
+
+You now need to decide on an installation location for both the executable as well as the configuration file.
+As an example, I'll be using `~/bin/lights-telegram/` as the install directory.
+
+You also need to decide which user will run the server.
+It needs write access to the directory / config file.
+In this example we'll also be using the current user, `thomas` in my case.
+
+Modify the username and paths in `lights-telegram.service` and the commands below as required.
+
+    CGO_ENABLED=0 go build
+    mkdir ~/bin/lights-telegram
+    cp lights-telegram ~/bin/lights-telegram/lights-telegram
+    cp config.yaml ~/bin/lights-telegram/config.yaml
+    cp lights-telegram.service /etc/systemd/system/lights-telegram.service
+    systemctl daemon-reload
+    systemctl enable --now lights-telegram
+
 ## Dependencies
 
  * [tgbotapi](https://pkg.go.dev/github.com/go-telegram-bot-api/telegram-bot-api/v5)
  * [mqtt](https://pkg.go.dev/github.com/eclipse/paho.mqtt.golang)
+ * [yaml](https://pkg.go.dev/gopkg.in/yaml.v3)
 
 ## License
 

lights-telegram.service

+[Unit]
+Description=Telegram bot MQTT interface
+Requires=network.target
+After=network.target
+After=syslog.target
+
+[Service]
+Type=simple
+Restart=always
+RestartSec=3
+User=thomas
+WorkingDirectory=/home/thomas/bin/lights-telegram
+ExecStart=/home/thomas/bin/lights-telegram/lights-telegram
+
+[Install]
+WantedBy=multi-user.target