See http://gohugo.io/sitemap.xml for an example:

<url>
<loc>http://gohugo.io/</loc>
<lastmod>2015-07-21T10:00:00+00:00</lastmod>
<priority>0</priority>
</url>

The homepage priority is always 0, even if a different value is set in the config file, e.g.:

[sitemap]
  priority = "0.5"

(As an aside, the sitemap config setting is not documented anywhere, the configuration page mentions it but doesn't define it).