doc: Update readme file

README.md

@@ -64,6 +64,8 @@ Note that for CSS media queries via [@custom-media](https://drafts.csswg.org/med
 }
 ```
 
+#### Plain CSS
+
 If you want to simply import the pre-bundled css variables without relying on a build process, you can import the minified `tokens.min.css` file as follows:
 
 ```html
@@ -73,6 +75,8 @@ If you want to simply import the pre-bundled css variables without relying on a
 />
 ```
 
+Note that the css above doesn't contain `@custom-media` variables because this directive is not yet supported by any modern browser.
+
 ### SCSS Usage
 
 You can import the SCSS variables into your files as follows: