Improve quick-start-guide

[PatrickThiem]

I tested this generator and had trouble work with it out of the box.

To be specific:

• ./gx-sd-generator.py --gaia-x --os-cloud=<os-cloud> --file=<file-name> - regarding os-cloud and file-name, here the guide needs to be more specific what is meant by that, explain the clouds.yaml file maybe or point to the OpenStack documentation - Done: Update README.md with missing content #57
• ./gx-sd-generator.py k8s - for k8s I was not able to get it working, I even think the command is not complete? In the help section there is a possible parameter listed for using the generator called "--kubeconfig" but it is not explained here
• get rid of descriptions like "Generate Gaia-X Self-Descriptions", as far as I know this tool is no longer developed to create self descriptions directly but to create credentials which can be used to create self-descriptions - See Rename repository title from self-description-generator to credential-generator #53