Add CONTRIBUTING note on local test config framework (#3438)

Co-authored-by: Logan Ward <WardLT@users.noreply.github.com>

CONTRIBUTING.rst

@@ -60,6 +60,10 @@ a suitable DFK is loaded with that configuration for each test.
 Tests which require their own specially configured DFK, or no DFK at all,
 should be labelled with ``@pytest.mark.local`` and can be run with
 ``--config local``.
+Provide the special configuration creating a ``local_config`` function
+that returns the required configuration in that test file.
+Or, provide both a ``local_setup`` function that loads the proper configuration
+and ``local_teardown`` that stops parsl.
 
 There is more fine-grained enabling and disabling of tests within the
 above categories: