We had to append the second part of this pattern in src/main/resources/site/site.xml in order to get the sitemap.xml to work. Otherwise we just to an error.
We did not find anything in README.md about this. Perhaps it should be added?
<mappings>
<mapping controller="/site/app.js" order="10">
<pattern invert="true">/r4xp5.*|.*/sitemap\.xml$</pattern>
</mapping>