Added license (MIT)

Instead of specifying each individual contributor, I think it would be better to link to the [GitHub contributors page](https://github.com/memeoirs/paymill-bundle/contributors). Or else we'll have to remember keeping this list up-to-date. [Other bundles](https://github.com/hwi/HWIOAuthBundle/blob/master/composer.json#L66) are using this approach.

```yml
{
    "name": "Contributors",
    "homepage": "https://github.com/memeoirs/paymill-bundle/contributors"
}
```