Update usage.rst
Update usage.rst

`on_delete` is required in Django 2.0

ref: https://docs.djangoproject.com/en/2.2/releases/1.9/#foreignkey-and-onetoonefield-on-delete-argument
takeflight/wagtailnews,takeflight/wagtailnews,takeflight/wagtailnews,takeflight/wagtailnews