[1.5.x] Fixed #19639 - Updated contributing to reflect model choices best practices.

  * ``def get_absolute_url()``

  * Any custom methods

* If ``choices`` is defined for a given model field, define the choices as

  a tuple of tuples, with an all-uppercase name, either near the top of

  the model module or just above the model class. Example::

* If ``choices`` is defined for a given model field, define each choice as

  a tuple of tuples, with an all-uppercase name as a class attribute on the

  model. Example::

    class MyModel(models.Model):

        DIRECTION_UP = 'U'

        DIRECTION_DOWN = 'D'

        DIRECTION_CHOICES = (

          ('U', 'Up'),

          ('D', 'Down'),

            (DIRECTION_UP, 'Up'),

            (DIRECTION_DOWN, 'Down'),

        )

Use of ``django.conf.settings``