Added initial version of contribution guidelines.

Largely based on discussions in #102 and [in the mailing list](https://groups.google.com/forum/#!topic/wsdl2phpgenerator/ViOv7C-SNTg) and [the contribution guidelines from FOSJsRoutingBundle](https://github.com/FriendsOfSymfony/FOSJsRoutingBundle/blob/master/CONTRIBUTING.md).