hook-up-a-custom-wp-feed.php

<?php 
add_action( 'after_setup_theme', 'advwp_create_upcoming_event_feed' );
function advwp_create_upcoming_event_feed() {    
    // Add_Feed() takes a "title" for the feed, and then a function to call
    // In this case, it calls a function that just points to the new
    // feed template.
    add_feed( 'upcoming', 'advwp_render_upcoming_event_feed');
}

function advwp_render_upcoming_event_feed() {
  require_once plugin_dir_path( __FILE__ ) . 'public/templates/feed-event.php';
}
?>