add suggested flattr button

docs/index.html

@@ -54,6 +54,14 @@
         Or join the <a href="http://librelist.com/browser/isso/">mailing list</a>,
         just send an email to <a href="mailto:isso@librelist.com">isso@librelist.com</a>.
     </p>
+    <h2>Contribute</h2>
+    <p>
+        <a href="{{ pathto('contribute') }}">Write Code</a>,
+        <a href="https://www.transifex.com/projects/p/isso/">Translate</a> or
+        <a href="https://flattr.com/thing/2059355/posativisso-on-GitHub">Flattr
+        <img src="{{ pathto('_static/flattr.png', 1) }}" alt="Flattr icon"/></a>.
+    </p>
+
     {% include "searchbox.html" %}
     </div>