Add documentation regarding TestCase vs TransactionalTestCase

[McPo]

Discovered that TestCases must use TransactionalTestCase, in order for the library to work. Might be worth explicitly mentioning this.

[Marviel]

Bump. Docs here: https://docs.djangoproject.com/en/2.0/topics/testing/tools/#django.test.TransactionTestCase

[vinnyrose]

also: https://docs.djangoproject.com/en/2.1/topics/db/transactions/#use-in-tests

[vinnyrose]

Section added to README to document this.

[github-actions]

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