text: Title: Add ability to document OPTIONS methods
  Body: Hey there, I'm trying to document a simple ModelViewSet on which I have overriden the OPTIONS method, but I can't figure out why the method does not appear in swagger.
  
  Can 
  
  ```python
  class ViewpointViewSet(viewsets.ModelViewSet):
      serializer_class = ViewpointSerializer
  
      @swagger_auto_schema(operation_description="OPTIONS /viewpoints/")
      def options(self, request, *args, **kwargs):
          if self.request.user.is_anonymous:
              raise PermissionDenied
          viewpoint_labels = ViewpointLabelSerializer(
              Viewpoint.objects.all(), many=True
          ).data
          return Response({
              'viewpoints': viewpoint_labels,
          })
  ```