Update README.md (#2004)

Adding the bash command for downloading the caddy.service file from the reposetory. Because it was easy to forget where you find it.

dist/init/linux-systemd/README.md

@@ -91,6 +91,7 @@ Install the systemd service unit configuration file, reload the systemd daemon,
 and start caddy:
 
 ```bash
+wget https://raw.githubusercontent.com/mholt/caddy/master/dist/init/linux-systemd/caddy.service
 sudo cp caddy.service /etc/systemd/system/
 sudo chown root:root /etc/systemd/system/caddy.service
 sudo chmod 644 /etc/systemd/system/caddy.service