Sitemaps.org wrote:You can specify the location of the Sitemap using a robots.txt file. To do this, simply add the following line:

Sitemap: <sitemap_location>The <sitemap_location> should be the complete URL to the Sitemap, such as: http://www.example.com/sitemap.xml

This directive is independent of the user-agent line, so it doesn't matter where you place it in your file. If you have a Sitemap index file, you can include the location of just that file. You don't need to list each individual Sitemap listed in the index file.