docs(cn): support global md config for attributes of external links (#358)

Author: ulivz

docs/guide/markdown.md

@@ -53,7 +53,7 @@ Outbound links automatically gets `target="_blank" rel="noopener noreferrer"`:
 - [vuejs.org](https://vuejs.org)
 - [VuePress on GitHub](https://github.com/vuejs/vuepress)
 
-You can customize the attributes added to external links by setting `config.markdown.externalLinks`. See more [here](/config/#markdown-externalLinks).
+You can customize the attributes added to external links by setting [config.markdown.externalLinks](../config/#markdown-externallinks).
 
 ## Front Matter
 

docs/zh/config/README.md

@@ -133,6 +133,13 @@ module.exports = {
 
 [markdown-it-anchor](https://github.com/valeriangalliat/markdown-it-anchor) 的选项。
 
+### markdown.externalLinks
+
+- Type: `Object`
+- Default: `{ target: '_blank', rel: 'noopener noreferrer' }`
+
+这个键值对将会作为特性被增加到是外部链接的 `<a>` 标签上，默认的选项将会在新窗口中打开一个该外部链接。
+
 ### markdown.toc
 
 - 类型: `Object`

docs/zh/guide/markdown.md

@@ -48,11 +48,13 @@
 
 ### 外部链接
 
-外部的链接将会被自动地设置为  `target="_blank"`:
+外部的链接将会被自动地设置为  `target="_blank" rel="noopener noreferrer"`:
 
 - [vuejs.org](https://vuejs.org)
 - [VuePress on GitHub](https://github.com/vuejs/vuepress)
 
+你可以自定义通过配置 [config.markdown.externalLinks](../config/#markdown-externallinks) 来自定义外部链接的特性。
+
 ## Front Matter
 
 VuePress 提供了对 [YAML front matter](https://jekyllrb.com/docs/frontmatter/) 开箱即用的支持: