title: GET Response with several models
body: <p>I have two ViewSets. They have unique serializer and model. For example, <code>CitiesViewSet</code> and <code>TypesViewSet</code>:</p>
  
  <p><code>CitiesViewSet</code> - </p>
  
  <pre><code>[{"name":"Moscow"},{"name": "Kazan"} etc...]
  </code></pre>
  
  <p><code>TypesViewSet</code> - </p>
  
  <pre><code>[{"id": 1, "name": "sample"}, {"id": 2, "name": "sample"} etc... ]
  </code></pre>
  
  <p>I want to combine this ViewSets to one GET Response. For example, I can do GET request and I will get looks like - </p>
  
  <pre><code> { "cities": 
    [
     {"name": "Moscow"},
     {"name": "Kazan"} etc...
    ], 
     "types": 
    [
     {"id": 1, "name": "sample"},
     {"id": 2, "name": "sample"} etc.
    ]
   }
  </code></pre>
  
  <p>How to create it?</p>
  
  <p><strong>UPD</strong>:</p>
  
  <pre><code>class CityViewSet(viewsets.ModelViewSet):
      pagination_class = None
      queryset = City.objects.all()
      serializer_class = CitySerializer
  
  
  class TypeViewSet(viewsets.ModelViewSet):
      pagination_class = None
      queryset = Type.objects.all()
      serializer_class = TypeSerializer
  </code></pre>
  
  <p>So I want to create a new viewset contains two viewsets (check example)</p>