docs: note that the version number must not be omitted

Author: haoqunjiang

README.md

@@ -15,3 +15,5 @@ Or, if you need to support IE11, you can create a Vue 2 project with:
 ```sh
 npm init vue@2
 ```
+
+Note that the version number (`@3` or `@2`) MUST NOT be omitted, otherwise `npm` may resolve to a cached and outdated version of the package.