templates: Ask for support after invoice payment.

pinaxcon/templates/registrasion/invoice/details.html

@@ -34,6 +34,12 @@
     This is a refunded registration summary for CopyleftConf 2020. It is provided for informational purposes only.
   {% elif invoice.is_paid %}
     This is a confirmed registration summary for CopyleftConf 2020.
+    </div>
+
+    <p class="text-success">Conservancy works year-round to grow the software
+      freedom movement and support alternatives to proprietary software.  Please
+      consider <b><a href="https://sfconservancy.org/supporter/">becoming a
+      Supporter</a></b> of our work!</p>
   {% endif %}
 {% endblock %}