Provide a working example with nginx-auto-ssl

[nguyenlamlll]

This PR provides an example for #48. I am not sure how you want to go with this @nezhar, so I leave them as commented lines.

To enable SSL for the WordPress site, one would need a domain (replace yourdomain.com with the actual domain) pointing to the server, uncomment these lines, and docker-compose up -d.

Instead of going directly to the Wordpress container, HTTP/S traffic will go first to nginx container, then go to WordPress. Because of that, wp container does not need to expose via port 80 now. Otherwise, port conflict will occur.