chore: update CONTRIBUTING.md for jib-extensions

[mpeddada1]

Following https://github.com/GoogleContainerTools/jib/pull/4192

Thank you for your interest in contributing! For general guidelines, please refer to the contributing guide.

Before submitting a pull request, please make sure to:

• [ ] Identify an existing issue to associate with your proposed change, or file a new issue.
• [ ] Describe any implementation plans in the issue and wait for a review from the repository maintainers.

Fixes # 🛠️