Document health_check.db's reliance on health_check_db_testmodel table

[rgmz]

The health_check.db check's reliance on health_check_db_testmodel can be surprising if you aren't using Django migrations.

I think it would be beneficial to call out in the install instructions.

e.g.,

If using the DB check, run migrations or create a table called health_check_db_testmodel

$ django-admin migrate

[frankwiles]

Not using Django Migrations with Django is pretty strange. Out of curiosity what are you using?