See: https://www.google.com/webmasters/sitemaps/docs/en/about.html
It's an XML format for listing pages on a site, it should be fairly easy to generate this with Petal, it's a simple format, eg:
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.google.com/schemas/sitemap/0.84">
<url>
<loc>http://www.yoursite.com/</loc>
<lastmod>2005-01-01</lastmod>
</url>
<url>
<loc>http://www.yoursite.com/foo/</loc>
<lastmod>2004-12-23</lastmod>
</url>
</urlset>
https://www.google.com/webmasters/sitemaps/docs/en/protocol.html
We might as well do this as /.sitemap.xml and perhaps a plain text version would also make sense:
https://www.google.com/webmasters/sitemaps/docs/en/faq.html#s9
What is a bit odd is that you have to sumbit the sitemap to google, they don't seem to intend to find them themselves:
https://www.google.com/webmasters/sitemaps/docs/en/faq.html#s4