[#116] Update README.md, add a line about cssCodeSplit (#117)

README.md

@@ -271,9 +271,11 @@ The `relativeCSSInjection` if configured to `true` will inject the CSS code of e
 **Set this option to `true` if you are using the multiple entry point option of Rollup.**
 _Future release can have an advanced behavior where this options will be configured to true automatically by sniffing user configurations._
 
-If a CSS code it's not injected inside some JS a warning will be showed.
+If a CSS chunk is generated that's not imported by any JS chunk, a warning will be shown.
 To disable this warning set `suppressUnusedCssWarning` to `true`.
 
+**For this feature to work, `build.cssCodeSplit` must be set to `true`**
+
 ## Contributing
 
 When you make changes to plugin locally, you may want to build the js from the typescript file of the plugin.