Add LinkedIn group to Participate menu (#2789)

Author: nlharris

_includes/header.html

@@ -139,6 +139,9 @@
                         <li>
                             <a class="dropdown-item" href="https://join.slack.com/t/obo-communitygroup/shared_invite/zt-3gs80gmrk-9yZT913GViLUg04pgE3wOA">OBO
                                 Community Slack channel</a>
+                        </li>
+                         <li>
+                            <a class="dropdown-item" href="https://www.linkedin.com/groups/14485140/">LinkedIn group</a>
                         </li>
                         <li>
                             <a class="dropdown-item" href="/docs/NewOBOFC.html">Join the OBO Operations Committee</a>