Add mobile sitemap to robots.txt

Added a new sitemap entry for the mobile version.

Author: 3code-type

robots.txt

@@ -1,4 +1,6 @@
 User-agent: *
 Disallow: /admin/
 
-Sitemap: https://3code-type.github.io/
+
+Sitemap: https://3code-type.github.io/
+Sitemap: https://3code-type.github.io/3code_mobile.html