Add link to GitHub docs on custom 404 pages

docs/tips.rst

@@ -88,6 +88,12 @@ by the ``ghp-import`` command) to the ``elemoine.github.io`` repository's
     To publish your Pelican site as User Pages, feel free to adjust the
     ``github`` target of the Makefile.
 
+Custom 404 Pages
+----------------
+
+GitHub Pages will display the custom 404 page described above, as noted in the
+relevant `GitHub docs <https://help.github.com/articles/custom-404-pages/>`_.
+
 Extra Tips
 ----------
 
@@ -125,3 +131,4 @@ Moreover, markup languages like reST and Markdown have plugins that let you
 embed videos in the markup. You can use `reST video directive
 <https://gist.github.com/dbrgn/2922648>`_ for reST or `mdx_video plugin
 <https://github.com/italomaia/mdx-video>`_ for Markdown.
+