Sitemap.xml file can be included in your robots.txt using this method. This will allow web crawlers to find your sitemap easily.
Referenced from: http://www.sitemaps.org/protocol.html#submit_robots
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 including the full URL to the sitemap:
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.
To submit your Sitemap using an HTTP request (replace <searchengine_URL> with the URL provided by the search engine), issue your request to the following URL:
For example, if your Sitemap is located at http://www.example.com/sitemap.gz, your URL will become:
URL encode everything after the /ping?sitemap=:
You can issue the HTTP request using wget, curl, or another mechanism of your choosing. A successful request will return an HTTP 200 response code; if you receive a different response, you should resubmit your request. The HTTP 200 response code only indicates that the search engine has received your Sitemap, not that the Sitemap itself or the URLs contained in it were valid. An easy way to do this is to set up an automated job to generate and submit Sitemaps on a regular basis.
Note: If you are providing a Sitemap index file, you only need to issue one HTTP request that includes the location of the Sitemap index file; you do not need to issue individual requests for each Sitemap listed in the index.