Docs: add more detail about how to run play mode for contribution (#14355)

* Docs: Add more detail about how to run play mode for contribution

* Update index.vue

.github/CONTRIBUTING.en-US.md

@@ -43,6 +43,8 @@ npm run dev
 # open http://localhost:8085
 ```
 
+> **Notice**: modify `examples/play/index.vue` file, use the component you contribute, then run `npm run dev:play`, go ahead http://localhost:8085, get result, more quickly and friendly.
+
 To build:
 
 ```shell

.github/CONTRIBUTING.es.md

@@ -47,6 +47,8 @@ npm run dev
 # abra http://localhost:8085
 ```
 
+> **Notice**: modify `examples/play/index.vue` file, use the component you contribute, then run `npm run dev:play`, go ahead http://localhost:8085, get result, more quickly and friendly.
+
 Para armar:
 
 ```

.github/CONTRIBUTING.zh-CN.md

@@ -41,6 +41,8 @@ npm run dev
 # open http://localhost:8085
 ```
 
+> **提示**：可以运行 `npm run dev:play`，修改 `examples/play/index.vue` 文件，调用你修改后的组件，仍然访问 http://localhost:8085，查看修改效果，更快更方便。
+
 打包代码：
 
 ```shell

examples/play/index.vue

 <template>
   <div style="margin: 20px;">
+    <el-input v-model="input" placeholder="请输入内容"></el-input>
   </div>
 </template>
 
 <script>
   export default {
+    data() {
+      return {
+        input: 'Hello Element UI!'
+      };
+    }
   };
 </script>