Add docs for enabling `rate-limit-nginx-module` in Docker

[kleisauke]

Context:

Oh, interesting.
I can see that it has been disabled, but is there any docs on how to enable it with the supervisord?
I suppose we need to remove the comment on that file, but also activate supervisord as well.
I think is worth to have the rate limiting even if that means having a little bit of heavy weight. If you are using it, there is a reason. :)

Originally posted by @planetahuevo in #168 (comment)