add BountySource link

docs/_themes/local/sidebarusefullinks.html

@@ -7,6 +7,7 @@
   <li><a href="https://pypi.python.org/pypi/borgbackup">PyPI packages</a></li>
   <li><a href="https://github.com/borgbackup/borg">GitHub</a></li>
   <li><a href="https://github.com/borgbackup/borg/issues">Issue Tracker</a></li>
+  <li><a href="https://www.bountysource.com/teams/borgbackup">Bounties &amp; Fundraisers</a></li>
   <li><a href="http://librelist.com/browser/borgbackup/">Mailing List</a></li>
 </ul>