Mention epubcheck

CONTRIBUTING.rst

@@ -3,8 +3,13 @@ report an issue on the issue tracker or hit me up on IRC, I'm always
 on ``#emacs``.  Patches are welcome, too, just fork, work on a
 separate branch and open a pull request with it.
 
+Invalid EPUB documents are *not* supported.  Please use epubcheck_ to
+validate yours when running into an error.
+
 In case the bug is specific to an EPUB document, please send it to me
 via email or upload it on an OCH and share the link with me.  I'll try
 my best to figure out the error, but chances are you can figure it out
 as well by using the source view (bound to ``v`` for the current
 document and ``V`` for the content file) to spot the problematic XML.
+
+.. _epubcheck: https://github.com/IDPF/epubcheck