DOC add a coc reference to the main page
<li><strong>Questions?</strong> See <a href="faq.html">FAQ</a> and <a href="https://stackoverflow.com/questions/tagged/scikit-learn">stackoverflow</a></li>
<li><strong>Mailing list:</strong> <a href="https://mail.python.org/mailman/listinfo/scikit-learn">scikit-learn@python.org</a></li>
<li><strong>Gitter:</strong> <a href="https://gitter.im/scikit-learn/scikit-learn">gitter.im/scikit-learn</a></li>
+ <li>Communication on all channels should respect <a href="https://www.python.org/psf/conduct/">PSF's code of conduct.</a></li>
</ul>
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