Improve docs with note about registry frontend web app

Hi,

I am the author of the [docker-registry-frontend](http://github.com/kwk/docker-registry-frontend) web app. It is a web application for browsing and manipulating a Docker registry by using the **Docker *Registry* API** and not the Docker *Remote* API. That is why I've create an extra page in the Docker documentation under "Reference -> Docker **Registry** API Client Libraries". This is where I started listing libraries and webapps just like on the "Reference -> Docker **Remote** API Client Libraries". The only exception is that those libraries deal with the Docker *Registry* API.

Please, accept my pull request or let me know what else I can do for the PR to be accepted. Since I only edited a few lines in the documentation, I consider my changes [an exception](https://github.com/docker/docker/blob/master/CONTRIBUTING.md#small-patch-exception) to the normal contribution guidelines. But let me know if you think different.

Thank you in advance
Konrad