Add info on FO processors to print production list

Fixes #36.

Signed-off-by: Roger Sheen <roger@infotexture.net>

index.html

@@ -177,6 +177,11 @@ <h2 class="featurette-heading">DITA infrastructure. <span class="text-muted">Pub
           </ul>
         </li>
       </ul>
+      <p>For PDF output, DITA-OT generates XSL Formatting Objects from DITA content. The interim XSL-FO format is passed
+      to an FO processor like the open-source <a href="https://xmlgraphics.apache.org/fop/" target="_blank">Apache™
+      FOP</a> <em>(included with DITA-OT)</em> or a commercial solution such as
+      <a href="https://www.antennahouse.com/antenna1/formatter/" target="_blank">Antenna House Formatter</a> or
+      <a href="http://www.renderx.com/tools/xep.html" target="_blank">RenderX XEP</a>, which renders the final PDF.</p>
       <p class="text-muted">To add your company to this list,
       <a href="https://github.com/dita-ot/dita-ot.github.io/issues/new" target="_blank">create an issue</a> or
       <a href="https://help.github.com/articles/using-pull-requests/" target="_blank">open a pull request</a>.</p>