I borrowed a startup script that was written for BTSync and modified its settings to work with syncthing. You can find the original script at http://bernaerts.dyndns.org/linux/75-debian/290-debian-server-btsync-permanent-peer .
I changed the parameters of the script to the following:
It’s been so long since I set this up I can’t remember if I had to manually create a service account. If I did, the command was
useradd -G sharing -m -d /home/syncthing -r -U syncthing
Lastly, make that script executable
chmod +x /etc/init.d/syncthing
update-rc.d syncthing defaults