[WIP] Add documentation on read the docs

Work in progress on the documentation:

* Use readthedocs (for versioning purpose)
* Separate Container / Image concept
* Add some cookbooks on how to do A ?, how to do B ? ....

This branch is currently building at: http://docker-php.readthedocs.org/en/feature-readthedocs (manual trigger for the moment)

ping @ubermuda :)
I'd rather use Composer's new feature: `composer require stage1/docker-php`

This is the new recommended way for any "Installation" section.