[new feature] Docs: 增加 contribute 指南 (#182)

* add contribute doc

* fix url path

* 文档格式调整

* readme 标题修改

* contribute guide for english

* contribute for english

* add pr & issue guildlines

* change title && add pr & issue in zh_CN.md

* add desc in zh_CN

* add zh-CN markdown

README.md

@@ -55,15 +55,9 @@ import 'vant/lib/vant-css/index.css';
 Vue.use(vant);
 ```
  
-## Development
+## How to contribute
 
-Start development mode:
-
-```shell
-npm run dev
-```
-
-Visit [http://localhost:8080](http://localhost:8080) to see an example of all components.
+Please make sure to read the [Contributing Guide](./docs/examples-docs/en-US/contribute.md) before making a pull request.
 
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