Document that this tool is not officially supported by Google

[tstromberg]

This is in-line with what I will be doing with our other GoogleContainerTools projects. Thoughts?

Fixes #<issue number>. in case of a bug fix, this should point to a bug and any other related issue(s)

Description

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- Skaffold config changes like
  e.g. "Add buildArgs to `Kustomize` deployer skaffold config."
- Bug fixes
  e.g. "Improve skaffold init behaviour when tags are used in manifests"
- Any changes in skaffold behavior
  e.g. "Artiface cachine is turned on by default."