adding details about versions to README

[mbkuhn]

Summary

Changing the README file to detail the intended versioning semantics for AMR-Wind

Pull request type

• [ ] Bugfix
• [ ] Feature
• [x] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):