Building the OpenAPI

Hello,

A month or so ago I had to patch your killbill integration, but I had serious issues in figuring out just how you have generated the code from the OpenAPI schema, what version of the openapi generator you used, what version of maven and java was used, etc...

Granted, I have not built things with java in a while, but I had to cheat a lot to make the progress I needed to.

So I would like to request you do a few things:

- Update the readme with explaining what version of all dev tools and java are required.
- Update the readme on how to properly generate the java class code from the OpenAPI.
- Explain why both gradle and maven are used and what is for what.
- Update the OpenAPI based on the latest API specs as there were several API's I had to hack in and create custom builds, to solve.
- Update the killbill plugin after to *use* the updated client.

Kudos.