Clarify the versioning policy in the contributing guide

[sagikazarmark]

Preflight Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

The contributing guide includes a section about bumping chart versions, but it doesn't offer any guidance on how.

Proposed Solution

Let's add a versioning policy to the contributing guide, something along these lines:

• patch version: very small changes, but mostly bug fixes only
• minor version: new features
• major version: breaking changes, discuss with the maintainers first

Alternatives Considered

No response

Additional Information

No response