Robots.txt is a specific format that is explained here [robotstxt.org....] Some search engines have made exceptions and will allow robots.txt to do things that are not officially allowed.
One example of an unofficial part of robots.txt that will work with search engines is placing the url of the sitemap file in the robots.txt file. A few search engines will follow that url to the sitemap. You should not include the actual sitemap data, only include the sitemap url.
I hope that helps you. Please let me know if you have another question.