docs: add 'extraWatchFiles' option to the documentation (#1593)

Author: YuYang019

Diff for: packages/docs/docs/config/README.md

@@ -113,6 +113,23 @@ vuepress dev docs --no-cache     # remove cache before each build.
 ```
 :::
 
+### extraWatchFiles
+
+- Type: `Array`
+- Default: `[]`
+
+Specify extra files to be watched.
+
+You can watch any file if you want. File changes will trigger `vuepress` rebuilding and real-time updates.
+
+``` js
+module.exports = {
+  extraWatchFiles: [
+    '.vuepress/foo.js', // Relative path usage
+    '/path/to/bar.js'   // Absolute path usage
+  ]
+}
+```
 
 ## Styling
 

Diff for: packages/docs/docs/zh/config/README.md

@@ -109,6 +109,24 @@ vuepress dev docs --no-cache     # 在每次构建前删除 cache
 ```
 :::
 
+### extraWatchFiles
+
+- 类型: `Array`
+- 默认值: `[]`
+
+指定额外的需要被监听的文件。
+
+你可以监听任何想监听的文件，文件变动将会触发 `vuepress` 重新构建，并实时更新。
+
+``` js
+module.exports = {
+  extraWatchFiles: [
+    '.vuepress/foo.js', // 使用相对路径
+    '/path/to/bar.js'   // 使用绝对路径
+  ]
+}
+```
+
 ## Styling
 
 ### palette.styl