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?

[codingjoe]

Hi there 👋, I don't mind adding more documentation. However, Django has checks and deployment checks that should prevent you from deploying with unapplied migrations. Do you want to provide a PR to extend the documentation yourself? Cheers, Joe