DOC Added Linkedin to social media section
<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><strong>Twitter:</strong> <a href="https://twitter.com/scikit_learn">@scikit_learn</a></li>
+ <li><strong>Linkedin:</strong> <a href="https://www.linkedin.com/company/scikit-learn">linkedin/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>
