Add information to how to configure test databases

docs/internals/testing.md

@@ -7,3 +7,8 @@ A good way to get started is to install the development version of the linter by
 
 To be able to fully test the linter, you will need both MySQL and PostgreSQL databases running.
 You can either tweak the ``tests/test_project/settings.py`` file to get your DB settings right, or to have databases and users corresponding to the default Travis users.
+
+## Database setup
+
+By default, the test Django project in the repository has multiple databases configured in order to make the Travis tests work.
+Do not hesitate to modify [the configured test databases](../../tests/test_project/settings.py).