message: Mention required material-ui v4 in README.md
diff: @@ -20,7 +20,7 @@ Accessible, customizable, delightful date & time pickers for [@material-ui/core]
  ### Installation
  -Please read the [migration guide](https://material-ui-pickers.dev/guides/upgrading-to-v3) if you are updating from v2
  +Note that this package reqiures `@material-ui/core` **v4**. It will not work with the old v3. Please read the [migration guide](https://material-ui-pickers.dev/guides/upgrading-to-v3) if you are updating from v2
  ```sh
  // via npm
  