Split documentation to multiple files in docs/ and use markdown

docs/internals/testing.md

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+# Testing
+
+The easiest way to run the tests is to invoke `tox` on the command line.
+
+You will need to install the test requirements, which can be found in the ``setup.py`` file.
+A good way to get started is to install the development version of the linter by doing ``pip install -e .[test]``.
+
+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.