index.html: Add Discourse link

Author: mkoeppe

index.html

@@ -12,7 +12,10 @@ <h1 style="font-size: 5.9vw";>
         <image width="1275" height="1754" xlink:href="art/Z12_128-150dpi.png">
         </image>
         <a xlink:href="https://github.com/passagemath/passagemath" xlink:title="passagemath">
-          <text x="637" y="170" text-anchor="middle">GitHub repository</text>
+          <text x="875" y="300" text-anchor="middle">GitHub repo</text>
+        </a>
+        <a xlink:href="http://passagemath.discourse.group/" xlink:title="passagemath">
+          <text x="235" y="350" text-anchor="middle">discourse</text>
         </a>
         <a xlink:href="docs/10.6" xlink:title="documentation">
           <text x="425" y="1290" text-anchor="middle">documentation 10.6.x</text>