There is an example config repo so you can see how to construct one and build from it. You can check that out here https://github.com/btopro/elmsln-config-example .

The Vagrant instance also has a config location that's managed on its own. Its only useful inside vagrant but if you want to see some ways you can add modules and change settings it's another good example: https://github.com/btopro/elmsln-config-vagrant/