Add missing link.

doc/_layouts/post.html

@@ -10,6 +10,7 @@ href="https://groups.google.com/group/capnproto-announce">Stay Updated</a></div>
   <a href="https://github.com/{{ page.author }}">{{ page.author }}</a>
   {% if page.author == 'kentonv' %}
   <span class="gplus-followbutton"><span class="g-follow" data-annotation="none" data-height="15" data-href="//plus.google.com/118187272963262049674" data-rel="author"></span></span>
+  <a href="https://www.gittip.com/kentonv/"><img class="gittip" src="{{ site.baseurl }}images/gittip.png" alt="Gittip"></a>
   {% endif %}
   on <span class="date">{{ page.date | date_to_string }}</span>
 </p>