Mark the wiki more-explicitely as being out of date.

Closes  #9689

docs/source/coredev/index.rst

@@ -1,27 +1,29 b''
 .. _core_developer_guide:
 
 =================================
 Guide for IPython core Developers
 =================================
 
 This guide documents the development of IPython itself.  Alternatively,
 developers of third party tools and libraries that use IPython should see the
 :doc:`../development/index`.
 
 
 For instruction on how to make a developer install see :ref:`devinstall`.
 
 .. toctree::
    :maxdepth: 1
 
    release_process
 
 
 Old Documentation
 =================
 
 Out of date documentation is still available and have been kept for archival purposes.
 
-Developers working on IPython should also consult the
-`developer information <https://github.com/ipython/ipython/wiki/Dev:-Index>`_
-on the IPython GitHub wiki.
+.. note::
+
+  Developers documentation used to be on the IPython wiki, but are now out of
+  date. The wiki is though still available for historical reasons: `Old IPython
+  GitHub Wiki.  <https://github.com/ipython/ipython/wiki/Dev:-Index>`_