Google allows sitemaps to be referenced in robots.txt

So far to let Google know about your sitemap you had to submit it  through Google Webmaster tools. Now, according to the Official Google Webmaster Central Blog you can just reference it in your robots.txt file:

Specifying the Sitemap location in your robots.txt file

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.

Now that's much easier.