Adds known classes which is required when de-serializing polymorphic structures
You need to use this method when your class interface does not tell the whole story about the objects it contains.
For example if you have property like
Item as Vehicle
But the actual data your putting in that property is Truck or Motorcycle, both of which inherit from Vehicle class, then you need to add Truck and Motorcycle as known classes since the serializer will not be able to get their class info from your class interface.