Give specific advice on using Kotlin build files

[mjaggard]

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

Please follow the guidelines below before opening an issue or a PR:

• Ensure the issue was not already reported.
• Create a new issue at https://github.com/GoogleContainerTools/jib/issues/new/choose if you are unable to find an existing issue addressing your problem. Make sure to include a title and clear description, as much relevant information as possible, and a code sample or an executable test case demonstrating the expected behavior that is not occurring.
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• Open a PR only if the issue is labeled with "Accepting Contributions", ensure the PR description clearly describes the problem and solution. Note that an open PR without an issues labeled with "Accepting Contributions" will not be accepted.
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Fixes #<issue_number_goes_here> 🛠️

[google-cla]

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