Add a page with a list libraries using HTTPlug

[Nyholm]

As discussed briefly on Slack. We would like a page similar to http://supervisord.org/plugins.html

We maybe also could mention the implementation. Are they using plugins? Custom plugins? MultipartStreamBuilder? Or maybe that is overkill.

Here is a list of projects using HTTPlug:
https://github.com/Payum/Payum
https://github.com/mailgun/mailgun-php
https://github.com/Happyr/LinkedIn-API-client
https://github.com/FriendsOfSymfony/FOSHttpCache
https://github.com/KnpLabs/php-github-api
https://github.com/flint/Stampie
https://github.com/SparkPost

[dbu]

it would be nice to show the logos of the most used "clients" on the home page of the doc, and maybe also in the README of Httplug for those that browse github. the full list could be on http://httplug.io/ maybe? its not really part of the documentation to me.

[sagikazarmark]

As far as I can see, this is fixed, isn't it?

[Nyholm]

Yes it is