Adapt documentation for ezplatform

[nicolas-bastien]

As ezplatform.com is opened to contribution, we should add a CONTRIBUTING.md file.
https://help.github.com/articles/setting-guidelines-for-repository-contributors/

Documentation should not copy/paste ezplatform but readme shoudl provides links to it.
All the following files provides confusion :

• GETTING_STARTED.md
• INSTALL.md
• RUNNING_BEHAT.md
• UPGRADE.md

Same remark for /doc folder it should contains project documentation.