chore: add how to customize extractors

Author: miyanokomiya

README.md

@@ -171,7 +171,32 @@ export default {
 }
 ```
 
+### Use custom extractors
+Each extention can be applied just one exractor.  
+If you apply a custom extractor to the extentions applied default extractor already, you have to override the default value.
 
+```js
+//nuxt.config.js
+export default {
+  modules: [
+    'nuxt-purgecss',
+  ],
+
+  purgeCSS: {
+    extractors: () => [
+      {
+        extractor: CustomExtractor,
+        extensions: ['html', 'vue', 'js']
+      },
+      {
+        extractor: CustomVueExtractor, // This cannot work, because above extractor is applied to 'vue' already.
+        extensions: ['vue']
+      }
+    ]
+  }
+}
+```
+ 
 ## Development
 
 - Clone this repository