Update README.md for archiving

[dwmcallister]

Archiving NGINX Unit repository

Proposed changes

Describe the use case and detail of the change. If this PR addresses an issue on GitHub, make sure to include a link to that issue using one of the supported keywords in this PR's description or commit message.

Checklist

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• [x ] I have read the contributing guidelines.
• [x ] I have signed the F5 Contributor License Agreement (CLA).
• [ x] If applicable, I have added tests that prove my fix is effective or that my feature works.
• [ x] If applicable, I have checked that any relevant tests pass after adding my changes.
• [ x] I have updated any relevant documentation (README.md and/or CHANGELOG.md).