Add gittip donation link

index.html

@@ -265,9 +265,10 @@
   <ul>
     <li>Donate
     (<span onclick="document.getElementById('paypal').submit();"
-    class="quasilink">Paypal</span>
-    or <span onclick="document.getElementById('bankinfo').style.display = 'block';"
-             class="quasilink">bank</span>)</li>
+    class="quasilink">Paypal</span>,
+    <span onclick="document.getElementById('bankinfo').style.display = 'block';"
+           class="quasilink">bank</span>, or
+    <a href="https://www.gittip.com/marijnh">Gittip</a>)</li>
     <li>Purchase <a href="#commercial">commercial support</a></li>
   </ul>