add motivation

Author: shigma

rfcs/001.blog-plugin.md

@@ -4,15 +4,18 @@
 
 # Summary
 
-A brand new `@vuepress/plugin-blog`, featured with tags, categories, drafting, pagination.
+- a new `@vuepress/plugin-blog`, featured with tags, categories, drafting, pagination
+- a `@vuepress/theme-plog`
 
 # Basic example
 
 See [detailed design](#detailed-design) section.
 
 # Motivation
 
-...
+By popular demand, VuePress will provide some official blog supports, including a blog plugin and an atomic blog theme.
+
+See: [#36](https://github.com/vuejs/vuepress/issues/36)
 
 # Detailed design