title: Serialize array/relation with attributes using ActiveModel::Serializer
body: <p>I want to serialize relation using Active Model Serializers and I want to set some 'global' attributes (e.g. count) for this relation:</p>
  
  <pre><code>{
    users: {
      total: 12,
      page: 2,
      users: [{}, {}, {}, ...]
    }
  }
  </code></pre>
  
  <p>How could I do that?</p>