Update migration guide

website/docs/migration-guides/yew/from-0_19_0-to-0_20_0.mdx

@@ -49,3 +49,12 @@ The reducer function can see all previous changes at the time they are run.
 `start_app*` has been replaced by `yew::Renderer`.
 
 You need to enable feature `render` to use `yew::Renderer`.
+
+## `ref` prop for Components
+
+Components no longer have a `ref` prop. Trying to add a node ref to a component
+will result in a compile error
+
+Previously node ref passed to a component was bound to the first element rendered by it.
+If this behavior is still desired, it is recommended to use add a `r#ref` field to the
+component's properties and bind it manually