Add note on installing via @7

Author: brophdawg11

docs/upgrading/v6.md

@@ -279,6 +279,8 @@ npm install react-router-dom@7
 
 Your app should continue to work but you'll get console warnings for importing from "react-router-dom". In v7 you can import everything directly from `"react-router"`.
 
+_Note: If you have issues with the above command, you may need to use the full `7.0.0-pre.N` version number because package managers may not always resolve `@7` to a prerelease since there is no stable 7.x release yet._
+
 👉 **Uninstall react-router-dom, install react-router**
 
 ```shellscript nonumber