Explain in the contributors guide what a "good first issue" is.

docs/CONTRIBUTING.rst

@@ -9,6 +9,13 @@ If you are looking for a good way to contribute to the Cython project, please
 * look through the `issues that are a good entry point for beginners <https://github.com/cython/cython/labels/good%20first%20issue>`_.
 * ask on the `core developers mailing list <https://mail.python.org/mailman/listinfo/cython-devel>`_ for guidance.
 
+Note that some (but not all) "good first issue"s also require an understanding of C
+and a bit of the CPython C-API – usually those that also have the ``Code Generation``
+label. We generally consider a ticket a "good first issue" if it has a limited scope
+that new contributors will have to learn about, e.g. only needs changes to the parser,
+the type analysis or the code generation, but does not require changes all across the
+compiler pipeline.
+
 If you have code that you want to contribute, please make sure that it
 
 * includes tests in the ``tests/`` directory (see the `Hacker Guide on Testing <https://github.com/cython/cython/wiki/HackerGuide#the-test-suite>`_)