How do I let search engines know about my Sitemap file?

to do it Ц register your website in webmaster services, provided by search engines (example: for Yandex or for Google). After registration you will be able to submit sitemap files from your account.
Another common way Ц include in robots.txt the following line:
Sitemap: http://website.tld/mysitemapfile.xml
If you need to provide several sitemap files, please add the same line for each file:
Sitemap: http://website.tld/mysitemapfile1.xml
Sitemap: http://website.tld/mysitemapfile2.xml
Sitemap: http://website.tld/mysitemapfile3.xml