s/image-sig/stackoverflow/

We don't use Image-SIG, it seems no one does anymore.

Author: aclark4life

index.html

@@ -92,7 +92,7 @@ <h3><a href=
       notes</a> and more.</p>
 
       <h3>Discussion</h3>
-      <p>Discussion occurs in <a href="https://github.com/python-pillow/Pillow/issues">GitHub Issues</a>, on the <a href="https://mail.python.org/mailman/listinfo/image-sig">mailing list</a> and on <a href="irc://irc.freenode.net#pil">IRC</a>. IRC conversations are <a href="http://chat-logs.dcpython.org/channel/pil">logged here</a>.
+      <p>Discussion occurs in <a href="https://github.com/python-pillow/Pillow/issues">GitHub Issues</a>, on the <a href="http://stackoverflow.com/questions/tagged/pillow">Stack Overflow</a> and <a href="irc://irc.freenode.net#pil">IRC</a>. IRC conversations are <a href="http://chat-logs.dcpython.org/channel/pil">logged here</a>.
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