docs: add migration guide

docs/src/pages/guides/migration-v4/migration-v4.md

@@ -45,6 +45,12 @@ yarn add @material-ui/core@next
 
 ## Handling breaking changes
 
+### non-ref-forwarding class components
+
+Support for these components in the `component` prop or as an immediate `children` has been dropped. If you were using `unstable_createStrictModeTheme` or didn't see any warnings related to `findDOMNode` in `React.StrictMode` then you don't need to do anything.
+Otherwise check out the ["Caveat with refs" section in our composition guide](/guides/composition/#caveat-with-refs) to find out how to migrate.
+This change affects almost all components where you're using the `component` prop or passing `children` to components that require `children` to be elements (e.g. `<MenuList><CustomMenuItem /></MenuList>`)
+
 ### BottomNavigation
 
 - typescript: The `event` in `onChange` is no longer typed as a `React.ChangeEvent` but `React.SyntheticEvent`.