Readme for option

There were infos missing about what is available in a template for options. I've added what I could find in the code + a few `???` where I'm not sure what they mean.
Yeah, but it's set in the code ... see here: https://github.com/vert-x3/vertx-codegen/blob/master/src/main/java/io/vertx/codegen/OptionsModel.java#L106