How to set the Fallback locales for Laravel 5 using laravel-translatable package

I'm trying to understand how to set the fallback locale for the laravel-translatable package to show the default translation when a translation has not been found.

In the documentation I've found this configuration example but I don't know where to put it.

return [
    'use_fallback' => true,
    'fallback_locale' => 'en',    
];

via Chebli Mohamed