docs(readme): clarify useable usage (#314)

Author: Lin84

README.md

@@ -88,6 +88,8 @@ import url from 'file.css'
 
 ### `Useable`
 
+The `style-loader` injects the styles lazily making them useable on-demand via `style.use()` / `style.unuse()`
+
 By convention the `Reference Counter API` should be bound to `.useable.css` and the `.css` should be loaded with basic `style-loader` usage.(similar to other file types, i.e. `.useable.less` and `.less`).
 
 **webpack.config.js**