docs: update contributing

Author: alexander-akait

README.md

@@ -1117,7 +1117,7 @@ module.exports = postcssCustomPlugin;
 We welcome all contributions!
 If you're new here, please take a moment to review our contributing guidelines before submitting issues or pull requests.
 
-[CONTRIBUTING](./.github/CONTRIBUTING.md)
+[CONTRIBUTING](https://github.com/webpack/postcss-loader?tab=contributing-ov-file#contributing)
 
 ## License