docs: update ignore instructions (#410)

README.md

@@ -315,6 +315,23 @@ module.exports = {
   ],
 };
 ```
+> ⚠️ Note that only relative path should be provided to ignore option, an example to ignore `src/assets/subfolder/ignorfile.js` :
+
+**webpack.config.js**
+
+```js
+module.exports = {
+  plugins: [
+    new CopyPlugin([
+      {
+        from: 'src/assets',
+        to: 'dest/',
+        ignore: ['subfolder/ingorefile.js'],
+      },
+    ]),
+  ],
+};
+```
 
 #### `flatten`