Extended the release notes for chainable Manager/QuerySet methods.

Calling custom ``QuerySet`` methods from the ``Manager``

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Historically, the recommended way to make reusable model queries was to create

methods on a custom ``Manager`` class. The problem with this approach was that

after the first method call, you'd get back a ``QuerySet`` instance and

couldn't call additional custom manager methods.

Though not documented, it was common to work around this issue by creating a

custom ``QuerySet`` so that custom methods could be chained; but the solution

had a number of drawbacks:

* The custom ``QuerySet`` and its custom methods were lost after the first

  call to ``values()`` or ``values_list()``.

* Writing a custom ``Manager`` was still necessary to return the custom

  ``QuerySet`` class and all methods that were desired on the ``Manager``

  had to be proxied to the ``QuerySet``. The whole process went against

  the DRY principle.

The :meth:`QuerySet.as_manager() <django.db.models.query.QuerySet.as_manager>`

class method has been added to :ref:`create Manager with QuerySet methods

<create-manager-with-queryset-methods>`.

class method can now directly :ref:`create Manager with QuerySet methods

<create-manager-with-queryset-methods>`::

    class FoodQuerySet(models.QuerySet):

        def pizzas(self):

            return self.filter(kind='pizza')

        def vegetarian(self):

            return self.filter(vegetarian=True)

    class Food(models.Model):

        kind = models.CharField(max_length=50)

        vegetarian = models.BooleanField()

        objects = FoodQuerySet.as_manager()

    Food.objects.pizzas().vegetarian()

Using a custom manager when traversing reverse relations

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~