feat: add migration guide > transition group root element

Author: naokie

src/guide/migration/transition-group.md

@@ -0,0 +1,40 @@
+---
+title: Transition Group Root Element
+badges:
+  - breaking
+---
+
+# {{ $frontmatter.title }} <MigrationBadges :badges="$frontmatter.badges" />
+
+## Overview
+
+`<transition-group>` no longer renders a root element by default, but can still create one with the `tag` prop.
+
+## 2.x Syntax
+
+In Vue 2, `<transition-group>`, like other custom components, needed a root element, which by default was a `<span>` but was customizable via the `tag` prop.
+
+```html
+<transition-group tag="ul">
+  <li v-for="item in items" :key="item">
+    {{ item }}
+  </li>
+</transition-group>
+```
+
+## 3.x Syntax
+
+In Vue 3, we have [fragment support](/guide/migration/fragments.html), so components no longer _need_ a root node. Consequently, `<transition-group>` no longer renders one by default.
+
+- If you already have the `tag` prop defined in your Vue 2 code, like in the example above, everything will work as before
+- If you didn't have one defined _and_ your styling or other behaviors relied on the presence of the `<span>` root element to work properly, simply add `tag="span"` to the `<transition-group>`:
+
+```html
+<transition-group tag="span">
+  <!-- -->
+</transition-group>
+```
+
+## See also
+
+- [Some transition classes got a rename](/guide/migration/transition.html)