docs: make clear about safelist

README.md

@@ -594,7 +594,7 @@ By default UnoCSS will scan for components files like: `.jsx`, `.tsx`, `.vue`, `
 Sometimes you might want have to use dynamic concatenations like:
 
 ```html
-<div class="p-${size}"></div>
+<div class="p-${size}"></div> <!-- this won't work! -->
 ```
 
 Due the fact that UnoCSS works in build time using static extracting, at the compile time we can't possibility know all the combination of the utilities. For that, you can configure the `safelist` option.