Order workflow mixin documentation

[rfleschenberg]

I talked to a user today who ran into the issue described by the traceback below.

As far as I understand, the problem is that he was using CommissionGoodsWorkflowMixin and PartialDeliveryWorkflowMixin at the same time, which is not supported. This is mentioned in the docstring of CommissionGoodsWorkflowMixin, but we should docment it more clearly in the to-be-written "manual setup" documentation.

Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/rene/.virtualenvs/stevensons/lib/python3.4/site-packages/django/core/management/__init__.py", line 353, in execute_from_command_line
    utility.execute()
  File "/home/rene/.virtualenvs/stevensons/lib/python3.4/site-packages/django/core/management/__init__.py", line 327, in execute
    django.setup()
  File "/home/rene/.virtualenvs/stevensons/lib/python3.4/site-packages/django/__init__.py", line 18, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/home/rene/.virtualenvs/stevensons/lib/python3.4/site-packages/django/apps/registry.py", line 108, in populate
    app_config.import_models(all_models)
  File "/home/rene/.virtualenvs/stevensons/lib/python3.4/site-packages/django/apps/config.py", line 202, in import_models
    self.models_module = import_module(models_module_name)
  File "/home/rene/.virtualenvs/stevensons/lib/python3.4/importlib/__init__.py", line 109, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 2254, in _gcd_import
  File "<frozen importlib._bootstrap>", line 2237, in _find_and_load
  File "<frozen importlib._bootstrap>", line 2226, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 1200, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 1129, in _exec
  File "<frozen importlib._bootstrap>", line 1471, in exec_module
  File "<frozen importlib._bootstrap>", line 321, in _call_with_frames_removed
  File "/tmp/stevensons-test/promotions/models.py", line 7, in <module>
    from products.models import Product
  File "/tmp/stevensons-test/products/models.py", line 26, in <module>
    from shop.models.defaults.order import Order  # noqa
  File "/home/rene/.virtualenvs/stevensons/lib/python3.4/site-packages/shop/models/defaults/order.py", line 10, in <module>
    class Order(order.BaseOrder):
  File "/home/rene/.virtualenvs/stevensons/lib/python3.4/site-packages/shop/models/order.py", line 115, in __new__
    raise ImproperlyConfigured(msg.format(b.__name__, ', '.join(TRANSITION_TARGETS.keys())))
django.core.exceptions.ImproperlyConfigured: Mixin class PartialDeliveryWorkflowMixin already contains a transition named 'ship_goods, pick_goods, pack_goods'