Update README.md

Author: mfranzke

packages/postcss-if-function/README.md

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-A [PostCSS](https://postcss.org/) plugin for transforming CSS `if()` functions into native CSS `@media` and `@supports` rules at build time.
+A [PostCSS](https://postcss.org/) plugin for transforming [CSS `if()` functions](https://developer.mozilla.org/en-US/docs/Web/CSS/if) into native CSS `@media` and `@supports` rules at build time.
 
 This plugin is part of the [css-if-polyfill](https://github.com/mfranzke/css-if-polyfill/tree/main/packages/css-if-polyfill/) project and provides build-time transformation of conditional CSS, eliminating the need for runtime JavaScript processing when using only `media()` and `supports()` functions.