Update Docs to include direct access to DjangoJob model

[irvinSing]

Using:
python version: Python 3.6.8
django version: 2.2.4

Not sure if it's actually documented anywhere, couldn't find it myself, but i managed to access the model directly by:

from django_apscheduler.jobstores import DjangoJob

obj = DjangoJob.objects.get(name=job_id)
obj.delete()

Reason that i had to do the above is because i wasn't able to remove the job by calling scheduler.remove_job('job_id').
This is despite the fact that i could clearly see the scheduled job in the Django admin interface with the 'name' field equivalent to 'job_id'.
However, if i call scheduler.get_jobs() or scheduler.print_jobs(), they show nothing at all.

I stored the 'job_id' separately in a different model using text field and extracted this from the separate model to input into the remove_job function. Not sure if this could have affected anything.. i doubt so.

In any case, i was able to delete the job directly by accessing the Django Job model directly.

Think this would be helpful information for anyone encountering difficulties removing job for any reason.

[jcass77]

django_apscheduler uses regular Django models and all of the standard Django ORM features are available - we unfortunately do not have formally structured documentation available at the moment where we can drill down to this level of detail.

I can't think of a reason why remove_job would not have worked for you, but it is possible that #79 could have muted some error condition without making you aware of it.