Document customElements option

[dhobi]

Documented the new option customElements merged with jquery-validation/jquery-validation#2493

Side note:
It might make sense to provide a docker-compose.yaml in this repository for an easier setup of the local wordpress instance.
I used this as a base which could be further improved to straightly provide the theme (git submodule?!) and the plugins (check in zip files in repository):

services:
  mariadb:
    image: mariadb:lts
    environment:
      MYSQL_ROOT_PASSWORD: rootpassword
      MYSQL_DATABASE: wordpress
      MYSQL_USER: wordpress
      MYSQL_PASSWORD: jqueryvalidation
    healthcheck:
      test: ["CMD", "healthcheck.sh", "--connect", "--innodb_initialized"]
      start_period: 10s
      interval: 10s
      timeout: 5s
      retries: 3
  wordpress:
    image: wordpress:4.7
    ports:
      - 8080:80
    environment:
      WORDPRESS_DB_HOST: mariadb:3306
      WORDPRESS_DB_NAME: wordpress
      WORDPRESS_DB_USER: wordpress
      WORDPRESS_DB_PASSWORD: jqueryvalidation
    depends_on:
      mariadb:
        condition: service_healthy