DOC Update note on code contribution: use issue tracker not mailing l…

…ist (#12917)

doc/developers/contributing.rst

@@ -169,10 +169,13 @@ Contributing code
 
 .. note::
 
-  To avoid duplicating work, it is highly advised that you contact the
-  developers on the mailing list before starting work on a non-trivial feature.
-
-  https://mail.python.org/mailman/listinfo/scikit-learn
+  To avoid duplicating work, it is highly advised that you search through the
+  `issue tracker <https://github.com/scikit-learn/scikit-learn/issues>`_ and
+  the `PR list <https://github.com/scikit-learn/scikit-learn/pulls>`_.
+  If in doubt about duplicated work, or if you want to work on a non-trivial
+  feature, it's recommended to first open an issue in
+  the `issue tracker <https://github.com/scikit-learn/scikit-learn/issues>`_
+  to get some feedbacks from core developers.
 
 How to contribute
 -----------------