docs: add custom PostCSS configuration file warning (#569)

Author: hdwatts

docs/guides/next.md

@@ -32,6 +32,8 @@ You can use PurgeCSS with Next.js by using the postCSS plugin in the Next.js con
 
 To customize the PostCSS configuration, create a postcss.config.js file in the root of your project.
 
+> Warning: When you define a custom PostCSS configuration file, Next.js completely disables the default behavior. Be sure to manually configure all the features you need compiled, including [Autoprefixer](https://github.com/postcss/autoprefixer). You also need to install any plugins included in your custom configuration manually, i.e. `npm install postcss-flexbugs-fixes postcss-preset-env`.
+
 Add PurgeCSS to the default configuration:
 
 ```js