README: Versioning information #1264

Problem description

stackit-cli should have a stable and deterministic versioning. Version 0.y.z indicates it can break every time and it might be confusing to ppl why a patch level (0.50.1) brings new features for example.

Proposed solution

• Make it more obvious in the README file what versioning schema is currently in place?
• Use for example semantic versioning and increase major version to 1 asap.