docs: update TODOs

Author: ulivz

README.md

@@ -37,6 +37,7 @@ https://vuepress.vuejs.org/
 
 VuePress is still a work in progress. There are a few things that it currently does not support but are planned:
 
+- Plugin support
 - Blogging support
 
 Contributions are welcome!

docs/guide/README.md

@@ -33,6 +33,7 @@ Each markdown file is compiled into HTML with [markdown-it](https://github.com/m
 
 VuePress is still a work in progress. There are a few things that it currently does not support but are planned:
 
+- Plugin support
 - Blogging support
 
 Contributions are welcome!

docs/zh/guide/README.md

@@ -31,6 +31,7 @@ VuePress 由两部分组成：一部分是支持用 Vue 开发主题的极简静
 
 VuePress 仍然处于开发中，这里有一些目前还不支持、但已经在计划中的特性：
 
+- 插件
 - 博客系统
 
 我们欢迎你为 VuePress 的开发作出贡献。