title: laravel scout add additional attribute (only in meilisearch)
body: <p>I try to migrate a SQL based search to meilisearch using laravel scout.</p>
  <p>At the moment the whole search should be migrated to meilisearch, including all filter and sorting options.</p>
  <p>A <code>product</code> has a relation to <code>feedbacks</code> (product model):</p>
  <pre><code>//returns all feedbacks for the product
  public function allFeedbacks()
  {
      return $this-&gt;hasMany('App\Models\Feedback');
  }
  </code></pre>
  <p>I would like to include the amount of feedbacks to meilisearch, but not the whole relation, since it's not required for sorting.</p>
  <p>How can I add additional fields to be index by meilisearch, without including a field into the mySQL database (<code>feedback_amount</code> f.e.)?</p>