Add a "fork me on Github" ribbon.

docs/_themes/attic/sidebarusefullinks.html

@@ -1,3 +1,6 @@
+<a href="https://github.com/jborg/attic"><img style="position: fixed; top: 0; right: 0; border: 0;"
+  src="http://s3.amazonaws.com/github/ribbons/forkme_right_gray_6d6d6d.png" alt="Fork me on GitHub"></a>
+
 <h3>Useful Links</h3>
 <ul>
   <li><a href="https://pypi.python.org/pypi/Attic">Attic @ PyPI</a></li>