Describe migration path for removed Themed.X as=Y pattern

MIGRATING.md

@@ -2,13 +2,16 @@
 
 ## v0.14
 
-- `@theme-ui/components` and other Theme UI packages no longer depend on
-  `@emotion/styled`.
+- `theme-ui`, `@theme-ui/components` and `@theme-ui/mdx` packages no longer
+  depend on `@emotion/styled`.
 
-- Previously deprecated `.withComponent` API was removed.
+- Previously deprecated `Component.withComponent` API was removed.
 
 - `as` prop was removed from Themed.X components from `@theme-ui/mdx`.
 
+  - All occurences of `<Themed.X as="element">` can be changed to
+    `<element sx={t => t.styles.X} />`.
+
 ## v0.13
 
 **Moved Emotion and `@mdx-js/react` to peerDependencies to solve context