Update README.md

Author: sagikazarmark

README.md

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+> [!WARNING]
+> This project is archived and no longer maintained. Consider using [`log/slog`](https://pkg.go.dev/log/slog) instead.
+>
+> Read more about why it was archived in [this post](https://sagikazarmark.com/blog/posts/less-is-more-archive-projects-for-a-better-open-source-ecosystem/).
+
 # Logur adapter for TEMPLATE
 
 [![GitHub Workflow Status](https://img.shields.io/github/workflow/status/logur/adapter-template/CI?style=flat-square)](https://github.com/logur/adapter-template/actions?query=workflow%3ACI)