Update README.md

Author: romainmenke

README.md

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+<div align="center">⚠️ PostCSS Custom Media was moved to <a href="https://github.com/csstools/postcss-plugins/tree/main/plugins/postcss-custom-media">@csstools/postcss-plugins</a>. ⚠️ <br>
+<a href="https://github.com/csstools/postcss-plugins/discussions/75">Read the announcement</a></div>
+
 # PostCSS Custom Media [<img src="https://postcss.github.io/postcss/logo.svg" alt="PostCSS" width="90" height="90" align="right">][postcss]
 
 [![NPM Version][npm-img]][npm-url]