docs: add note about gitbook

docs/guide/README.md

@@ -48,3 +48,7 @@ Both are great projects and also Vue-powered. Except they are both completely ru
 ### Hexo
 
 Hexo has been serving the Vue docs well - in fact, we are probably still a long way to go from migrating away from it for our main site. The biggest problem is that its theming system is very static and string-based - we really want to leverage Vue for both the layout and the interactivity. Also, Hexo's markdown rendering isn't the most flexible to configure.
+
+### Gitbook
+
+We've been using Gitbook for most of our sub project docs. The primary problem with Gitbook is that its development reload performance is intolerable with a large amount of files. The default theme also has a pretty limiting navigation structure, and the theming system is, again, not Vue based. The team behind GitBook is also more focused on turning it into a commercial product rather than an open-source tool.

docs/zh/guide/README.md

@@ -47,3 +47,6 @@ VuePress 能做的事情，Nuxt 理论上确实能够胜任，但 Nuxt 是为构
 
 Hexo 一直驱动着 Vue 的文档 —— 事实上，在把我们的主站从 Hexo 迁移到 VuePress 之前，我们可能还有很长的路要走。Hexo 最大的问题在于他的主题系统太过于静态以及过度地依赖纯字符串，而我们十分希望能够好好地利用 Vue 来处理我们的布局和交互，同时，Hexo 的 Markdown 渲染的配置也不是最灵活的。
 
+### Gitbook
+
+我们的子项目文档一直都在使用 Gitbook。Gitbook 最大的问题在于当文件很多时，每次编辑后的重新加载时间长得令人无法忍受。它的默认主题导航结构也比较有限制性，并且，主题系统也不是 Vue 驱动的。Gitbook 背后的团队如今也更专注于将其打造为一个商业产品而不是开源工具。