docs: add deprecation notice to README

This package is no longer maintained.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Patrick Heneise

README.md

@@ -1,9 +1,7 @@
-![Test](https://github.com/zentered/frontformatter/workflows/Test/badge.svg)
-![Semantic Release](https://github.com/zentered/frontformatter/workflows/Semantic%20Release/badge.svg?branch=main)
-[![view on npm](https://img.shields.io/npm/v/frontformatter.svg)](https://www.npmjs.org/package/frontformatter)
-
 # Front Formatter
 
+> **Deprecated:** This package is no longer maintained.
+
 If you work with [hugo](https://gohugo.io), [11ty](https://www.11ty.dev) or any other static site generator, you'll likely end up with a lot of Markdown files after a while.
 
 This script helps to migrate, add or remove properties from Markdwon frontmatter.