Add a link to Individual membership in the site footer

This is a companion change to go along with the footer updates made to
the main djangoproject.com website at django/djangoproject.com#1726.

trac-env/templates/site_footer.html

@@ -22,6 +22,7 @@ <h2>Get Involved</h2>
             <li><a href="https://docs.djangoproject.com/en/dev/internals/contributing/">Contribute to Django</a></li>
             <li><a href="https://docs.djangoproject.com/en/dev/internals/contributing/bugs-and-features/#reporting-security-issues">Submit a Bug</a></li>
             <li><a href="https://docs.djangoproject.com/en/dev/internals/security/">Report a Security Issue</a></li>
+            <li><a href="https://www.djangoproject.com/foundation/individual-members/">Individual membership</a></li>
           </ul>
         </div>