[1.4.X] Fixed #13586 - Added an example of how to connect a m2m_changed signal handler.

        # ...

        toppings = models.ManyToManyField(Topping)

If we would do something like this:

If we connected a handler like this::

    def toppings_changed(sender, **kwargs):

        # Do something

        pass

    m2m_changed.connect(toppings_changed, sender=Pizza.toppings.through)

and then did something like this::

    >>> p = Pizza.object.create(...)

    >>> t = Topping.objects.create(...)

    >>> p.toppings.add(t)

the arguments sent to a :data:`m2m_changed` handler would be:

the arguments sent to a :data:`m2m_changed` handler (``topppings_changed`` in

the example above) would be:

==============  ============================================================

Argument        Value