Added contributing informations

| Q               | A
| --------------- | ---
| Bug fix?        | no
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Tests pass?     | yes
| Related tickets |
| License         | GNU GPL v3

It's base on the [Symfony contributing standard][0].

[0]: http://symfony.com/doc/current/contributing/code/patches.html