docs: add roadmap link

Author: meteorlxy

README.md

@@ -10,6 +10,8 @@
 
 For users from VuePress v1, please check out the [migration guide](https://vuepress2.netlify.app/guide/migration.html).
 
+For users who want to follow our progress or contribute to this project, please check out the [roadmap](https://github.com/vuepress/vuepress-next/discussions/68).
+
 ## Documentation
 
 https://vuepress2.netlify.app