Update Contributing.md

[pascalchevrel]

• Minimum PHP is now 5.6
• Launching tests is now start.sh -tests
• Setting the remote should use the name "upstream" as it is the general convention for Github forking
• More?

I would suggest to link to Francesco's git tuto

Contribution tips should be moved from README.md to CONTRIBUTING.md. And we should link to CONTRIBUTING.md inside the README In the Contribution tips