Add an example of implementation validation rules in a model extending about docs

[igor-tv]

Yes thank you. This is what I need.

[daftspunk]

I've added this to the docs. Is it what you're after?

---

To add model validation for introduced fields, hook into the beforeValidate event and throw a ValidationException exception.

User::extend(function ($model) {
    $model->bindEvent('model.beforeValidate', function () use ($model) {
        if (!$model->billing_first_name) {
            throw new \ValidationException(['billing_first_name' => 'First name is required']);
        }
    });
});