Document that 'mixpanel.tasks' must be added to the "include" parameter of the Celery app instantiation

In order to get my Celery worker to recognize `EventTracker` as a valid task, I had to update this:

```
celery = Celery(
    'myproject',
    broker=settings.REDIS_URL,
    include=['myproject.tasks'],
    )
```

to this;

```
celery = Celery(
    'myproject',
    broker=settings.REDIS_URL,
    include=['myproject.tasks', 'mixpanel.tasks'],
    )
```

It probably also would have worked if I'd imported ``mixpanel.tasks`` in my ``myproject.tasks`` module, but there was no obvious reason to do this.

In any case, it doesn't seem like this issue is mentioned at all in the documentation (unless I missed it).