Enhance CodeGen README

Besides working examples (see https://github.com/eclipse-thingweb/td-tools/pull/82) I suggest adding what are the prerequisites, what file do I need to store and what to run to make it bulletproof. Similar to what we have in node-wot (see https://github.com/eclipse-thingweb/node-wot/blob/master/packages/binding-http/README.md#client-example)

Moreover, I am not exactly sure how I can run the CI. In my quick test I failed.

Start with the CodeGen README and compare its examples with the working example in pull request #82 and the client example in node-wot's binding-http README. Document prerequisites, the Thing Description file to store, commands to run, and how to run CI; done means a newcomer can follow the README successfully and the CI steps are clear.

Written by the indexing model from the issue text.