Document new field API in release notes

  will still work, but that method name is deprecated and you should change

  it as soon as possible (nothing more than renaming is required).

New method on Field subclasses

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

To help power both schema migrations and composite keys, the Field API now

has a new required method: ``deconstruct()``.

This method takes no arguments, and returns a tuple of four items:

* ``name``: The field's attribute name on its parent model, or None if it is not part of a model

* ``path``: A dotted, Python path to the class of this field, including the class name.

* ``args``: Positional arguments, as a list

* ``kwargs``: Keyword arguments, as a dict

These four values allow any field to be serialized into a file, as well as

allowing the field to be copied safely, both essential parts of these new features.

This change should not affect you unless you write custom Field subclasses;

if you do, you may need to reimplement the ``deconstruct()`` method if your

subclass changes the method signature of ``__init__`` in any way. If your

field just inherits from a built-in Django field and doesn't override ``__init__``,

no changes are necessary.

If you do need to override ``deconstruct()``, a good place to start is the

built-in Django fields (``django/db/models/fields/__init__.py``) as several

fields, including ``DecimalField`` and ``DateField``, override it and show how

to call the method on the superclass and simply add or remove extra arguments.

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

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