docs: add disclaimer

Author: haoqunjiang

README.md

@@ -2,6 +2,10 @@
 
 A Vue CLI plugin for trying out the Vue 3 alpha.
 
+This is for preview purposes only. There might be bugs and undocumented behavior differences from v2, which are expected.
+
+Also note that if you are using VSCode, Vetur isn't updated to take advantage of Vue 3's typing yet so intellisense in Vue files may not be fully functional (especially in templates).
+
 ## Usage
 
 ```sh