massPrunable must be changed + Missing tests

[afpinedac]

I tried to send PR but it was blocked.

I think the right Implementation is

protected function massPrunable()
{
return static::where('created_at', '<=', now()->subDays(config('deleted-models.prune_after_days')));
}

Also, the CONTRIBUTING.md file is missing. How can we do that?

[freekmurze]

The tests prove that the current behaviour is working.