French translation

Hi,

I've just translated this package's `errors.php` and `validation.php` files to French for a project. Would you be interested in a PR for this? I can see there was one [for Dutch](https://github.com/cloudcreativity/laravel-json-api/pull/563) a few months ago, but just wanna make sure you'd actually be interested first.

Cheers,

Yannick