Initial contributing guide, based on Symfony

| Q                    | A
| ------------------| ---
| Doc fix?          | no
| New docs?     | yes
| Fixed tickets   | none

Hey, contributing to Sylius is in many ways similar to Symfony, so I asked the docs team if it is fine with them to take Symfony Contributing Guide and adapt it to our needs.

The BDD part needs some more work, but I hope to release it today. If you have something to add, please let me know!