Add the Yoast SEO sitemap index to the WordPress generated robots.txt file.

yoast_seo_robots_add_sitemap.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/* Add Sitemap Index To Generated Robots.txt
 * Credit: Ben https://github.com/retlehs
 * Last Tested: Mar 13 2017 using Yoast SEO 4.4 on WordPress 4.7.3
 */

add_action('do_robots', 'custom_robotstxt');

function custom_robotstxt() {
  echo 'Sitemap: ' . get_home_url() . '/sitemap_index.xml';
}