Create contribution guidelines

[lforite]

Hello guys,

As an extensive user of Play and Play and Play Json, I would like to start contributing to those libraries. Unfortunately, for Play Json there are no contribution guidelines.

I think the README should include contributing guidelines, and it might refer to the regular (Play ones)[https://www.playframework.com/contributing]

[cchantep]

Would rather add a CONTRIBUTING.md as https://github.com/playframework/playframework/blob/master/CONTRIBUTING.md