Add a section to DjangoItem docs page regarding setting up Django's settings.

Based on the suggestions of Pablo in the mailing list regarding to an [answer](http://stackoverflow.com/questions/19068308/access-django-models-with-scrapy-defining-path-to-django-project/19073347#19073347) in SO, this pull request adds a short explanation on how to set up the django settings to be able to use the models and avoid the highly frequent `ImportError` exception.