add stub entry for Tabs+Pills changes to the migration guide

docs/migration.md

@@ -30,6 +30,8 @@ This table shows the style changes between v3.x.x and v4.0.0.
 | Pager's `.previous` | `.pager-prev` |
 | Pager's `.next` | `.pager-next` |
 | Carousel's `.item` | `.carousel-item` |
+| Tabs | |
+| Pills | |
 
 ## What's new
 We've added new components and changed some existing ones. Here are the new or updated styles.