CKEditor integration doc

At La Coopérative des Tilleuls we've wrote [a bundle](https://github.com/coopTilleuls/CoopTilleulsCKEditorSonataMediaBundle) to integrate SonataMedia with CKEditor.

It allows to browse, select and upload SonataMedia files and images directly from the CKEditor UI.
It integrates well with IvoryCKEditorBundle and SonataFormatterBundle.

If @rande is interested we are OK to move this bundle to the Sonata Project.

This PR update the doc of SonataMedia to explain how to use this new bundle and update the "suggest" field of `composer.json`.