doc(readme): Add note about fixed mode w/major version zero

[0livare]

Description

Add a note to the README about the lerna publish functionality when using a version number where the major revision is 0.

This fixes #2487

I'm not sure if this note describes what "should" happen, but it accurately
describes the current functionality.

Motivation and Context

The current Fixed/Locked mode documentation is not accurate when using a major version 0. This ammendment now makes the documentation accurate for both zero and non-zero major version revisions.

How Has This Been Tested?

This cannot be tested.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[evocateur]

Thanks for your patience!