docs/usage.txt has import error

[adamn]

This needs to be added to the example in docs/usage.txt for creating notification events on syncdb:

Add:
from django.db.models.signals import post_syncdb

And then change on line 12:
signals.post_syncdb.connect(create_notice_types, sender=notification)
To:
post_syncdb.connect(create_notice_types, sender=notification)

[adamn]

Addressed here:

https://github.com/AdamN/django-notification/commit/cc5d3563cee49e36b85173d419b768ce39110e1e