Things have Changed!
Google has discontinued support for Geo sitemaps, or more specifically, Geo XML tags in sitemaps.
Previously, you could create your KML file (containing your Geo information), upload to your server and create a separate sitemap file containing Geo specific XML tags (no longer supported), or add the Geo specific tags to your normal sitemap. These Geo specific XML tags are no longer supported, but you can still add the url of your KML file to your ordinary sitemap, just as you would add a normal page url.
Learn about Creating and Sharing KML Files Read Google no longer supports Geo-specific XML tags in sitemaps (What is KML?)
The basic steps for publishing your geo content are as follows:
1: Create the KML content. Add attribution tags, which will appear in the Google Search results for your content.
2: Upload the KML file to your web server.
3: Create the Geo Sitemap file. Copy this file to the root directory of your website. (discontinued)
4: Submit your Geo Sitemap to Google. (discontinued)
3: Add the KML file url to your sitemap.