Chore: update contribution guide (#9803)

.github/CONTRIBUTING.en-US.md

@@ -35,7 +35,7 @@ We are excited that you are interested in contributing to Element. Before submit
 - Merging a PR takes two maintainers: one approves the changes after reviewing, and then the other reviews and merges.
 
 ## Prerequisites
-`Node.js 4+` and `NPM 3+` are required.
+`Node.js 4+`, `yarn` and `npm 3+` are required. Note: we use yarn to lock dependency versions, so you should install dependencies using `yarn` instead of `npm install`.
 ```shell
 git clone git@github.com:ElemeFE/element.git
 npm run dev
@@ -43,16 +43,6 @@ npm run dev
 # open http://localhost:8085
 ```
 
-For Chinese users, [yarn](https://github.com/yarnpkg/yarn) with taobao registry is recommended if the dependency installation is slow.
-```shell
-npm i yarn -g
-yarn config set registry https://registry.npm.taobao.org
-yarn
-npm run dev
-
-# open http://localhost:8085
-```
-
 To build:
 
 ```shell

.github/CONTRIBUTING.es.md

@@ -38,7 +38,7 @@ Estamos orgullosos de que usted esta interesado en contribuir al proyecto `Eleme
 
 
 ## Requerimientos Técnicos 
-`Node.js 4+` y `NPM 3+` son requisitos
+`Node.js 4+`, `yarn` y `npm 3+` son requisitos. Nota: Usamos yarn para bloquear versiones de dependencias, por lo que debería instalar dependencias usando `yarn` en lugar de `npm install`.
 .
 ```shell
 git clone git@github.com:ElemeFE/element.git
@@ -47,17 +47,6 @@ npm run dev
 # abra http://localhost:8085
 ```
 
-Para usuarios chinos, [yarn](https://github.com/yarnpkg/yarn) con `taobao registry` es recomendado, sí la instalación de dependencias es lenta.
-
-```shell
-npm i yarn -g
-yarn config set registry https://registry.npm.taobao.org
-yarn
-npm run dev
-
-# abra http://localhost:8085
-```
-
 Para armar:
 
 ```

.github/CONTRIBUTING.zh-CN.md

@@ -33,7 +33,7 @@ Element UI 的成长离不开大家的支持，如果你愿意为 Element UI 贡
 - 合并代码需要两名维护人员参与：一人进行 review 后 approve，另一人再次 review，通过后即可合并。
 
 ## 开发环境搭建
-首先你需要 Node.js 4+ 和 NPM 3+
+首先你需要 Node.js 4+，yarn 和 npm 3+。注意：我们使用 yarn 进行依赖版本的锁定，所以请不要使用 `npm install` 安装依赖。
 ```shell
 git clone git@github.com:ElemeFE/element.git
 npm run dev
@@ -41,17 +41,7 @@ npm run dev
 # open http://localhost:8085
 ```
 
-如果国内用户觉得安装慢可以使用 [yarn](https://github.com/yarnpkg/yarn) 搭配 taobao registry
+打包代码：
-```shell
-npm i yarn -g
-yarn config set registry https://registry.npm.taobao.org
-yarn
-npm run dev
-
-# open http://localhost:8085
-```
-
-To build:
 
 ```shell
 npm run dist

package.json

@@ -11,7 +11,7 @@
   ],
   "typings": "types/index.d.ts",
   "scripts": {
-    "bootstrap": "yarn || npm i",
+    "bootstrap": "yarn",
     "build:file": "node build/bin/iconInit.js & node build/bin/build-entry.js & node build/bin/i18n.js & node build/bin/version.js",
     "build:theme": "node build/bin/gen-cssfile && gulp build --gulpfile packages/theme-chalk/gulpfile.js && cp-cli packages/theme-chalk/lib lib/theme-chalk",
     "build:utils": "cross-env BABEL_ENV=utils babel src --out-dir lib --ignore src/index.js",