Add breaking changes to migration guide

docs/data/system/migration/migration-v5/migration-v5.md

@@ -23,3 +23,19 @@ The steps you need to take to migrate from MUI System v5 to v6 are described b
 This list is a work in progress.
 Expect updates as new breaking changes are introduced.
 :::
+
+### Root code is now ESM
+
+The ESM code, previously under the `esm/` build, has been moved to the root of the package.
+The CommonJS code, previously on the root, has been moved to the `node/` build.
+
+:::info
+This is an intermediate step to prepare for adding the `exports` field to the `package.json` file.
+If you have trouble using this new structure, please wait for the future update which adds the `exports` field.
+You can follow progress on https://github.com/mui/material-ui/issues/30671.
+:::
+
+### GridProps type
+
+The `cssGrid` function's `GridProps` type has been renamed to `CssGridProps`.
+This is to avoid collision with the `GridProps` type corresponding to the `Grid` component props.