Is the front end going to be using Slim? If it will, then everything will go through the front controller (
index.php) anyway, so just build the routes accordingly.
$app->get('/', // example.com/
$app->get('/contact', // example.com/contact
If it is outside of Slim then I'd probably put your front controller in a directory called API. (Or more likely I would use a different subdomain entirely.)