Notification When Post Approved

You can use publish_post action hook for notifying post author.

function wpcs_author_notification( $post_id ) {

   $post = get_post( $post_id );
   $author = get_userdata( $post->post_author );

   $message = "Hi " . $author->display_name . ", Your post, " . $post->post_title . " has just been published at " . get_permalink( $post_id );

   wp_mail( $author->user_email, "Your article is published", $message );

}

add_action( 'publish_post', 'wpcs_author_notification' );

This function will notify author for published post. You can modify message in above code.