<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def contribute_to_class(model_class, name='slots', descriptor=None):
 """ Function that adds a description to a model Class. :param model_class: The model class the descriptor is to be added to. :param name: The attribute name the descriptor will be assigned to. :param descriptor: The descriptor instance to be used. If none is specified it will default to ``icekit.plugins.descriptors.PlaceholderDescriptor``. :return: True """

    rel_obj = descriptor or PlaceholderDescriptor()
    rel_obj.contribute_to_class(model_class, name)
    setattr(model_class, name, rel_obj)
    return True