fix($default-theme): header should be fixed

Author: ulivz

packages/docs/docs/README.md

@@ -27,9 +27,16 @@ footer: MIT Licensed | Copyright © 2018-present Evan You
 
 ### As Easy as 1, 2, 3
 
+
+::: warning 
+Note that this is the documentation of 1.x.x, if you are a 0.x.x user please go to: [https://v0.vuepress.vuejs.org.](https://v0.vuepress.vuejs.org).
+Since 1.x.x is still in the `alpha` stage, all APIs can happen before we release the rc version, so do not use in production yet unless you are adventurous.
+:::
+
 ``` bash
 # install
-yarn global add vuepress # OR npm install -g vuepress
+yarn global add vuepress@next 
+# OR npm install -g vuepress@next
 
 # create a markdown file
 echo '# Hello VuePress' > README.md