Add Custom Post Type To RSS Feed

add_filter( 'pre_get_posts', 'add_cpt_to_loop_and_feed' );
/**
 * @author    Brad Dalton
 * @example   http://wpsites.net/wordpress-tips/add-custom-post-type-to-rss-feed/
 * @copyright 2014 WP Sites
 */
function add_cpt_to_loop_and_feed( $query ) {

	if ( is_home() && $query->is_main_query() || is_feed() )
		$query->set( 'post_type', array( 'post', 'your-cpt' ) );

	return $query;
}