Add notice to CSS variables guide

Author: kristerkari

docs/setup-css-variables.md

@@ -1,5 +1,7 @@
 ## Setup CSS variables support
 
+_! Please make sure that you are using the latest version of Sass, LESS, or Stylus transformer before doing the setup. !_
+
 Using [postcss-css-variables](https://github.com/MadLittleMods/postcss-css-variables#readme) you can use most of CSS variables features, including selector cascading with some caveats, because this can only see the CSS, not the potentially dynamic HTML and DOM the CSS is applied to.
 
 If you are already using `react-native-css-transformer`, then need to switch to use `react-native-postcss-transformer` (please refer to the setup documentation) and add a PostCSS config with [postcss-css-variables](https://github.com/MadLittleMods/postcss-css-variables#readme) plugin.