update doc for FAQ

Author: iamcco

doc/mkdp.txt

@@ -65,6 +65,18 @@ close preview window:
     nmap <silent> <F9> <Plug>StopMarkdownPreview    " for normal mode
     imap <silent> <F9> <Plug>StopMarkdownPreview    " for insert mode
 
+FAQ:
+
+Q: the firefox preview window didn't close when leave the markdown file in vim
+
+A: if you want the plugin auto close the preview window on firefox, you have to do some config:
+
+1. open firefox
+2. type `about:config` in the address bar and press Enter key
+3. search `dom.allow_scripts_to_close_windows` item and set the value to `true`
+
+> you have to know what will happend when you do the config above
+
 ================================================================================
 
 说明                                                   *mkdp-introduction-zh*
@@ -122,3 +134,15 @@ close preview window:
     imap <silent> <F8> <Plug>MarkdownPreview        " 插入模式
     nmap <silent> <F9> <Plug>StopMarkdownPreview    " 普通模式
     imap <silent> <F9> <Plug>StopMarkdownPreview    " 插入模式
+
+FAQ:
+
+Q: 在火狐浏览器中，预览页面不能自动关闭
+
+A: 如果需要在火狐浏览器中启用自动关闭预览窗口，需要相关设置：
+
+1. 打开火狐浏览器
+2. 地址栏敲入 `about:config` 然后回车
+3. 搜索 `dom.allow_scripts_to_close_windows` 选项，并设置 value 为 `true`
+
+> 如果你打算设置上面的配置，你应该知道该配置会产生什么影响