Add Sidebar changes to migration Docs

Per Issue vuepress#1238 added some highlights for theme changes related to sidebars that we had to dig for in theme config.

docs/guide/migration.md

@@ -275,6 +275,35 @@ Default theme config has been changed a lot. You'd better check the config refer
 
 See [Default Theme > Config](../reference/default-theme/config.md).
 
+##### Some theme notable changes:
+
+Sidebar children no longer support arrays in arrays.
+
+  Old V1
+  ```js
+   {
+       title: 'Configuring Atlantis',
+       path: '/your/path'
+       collapsible: true,
+       children: [
+           ['configuring-atlantis', 'Overview'],
+  ```
+  New V2 format
+  ```js
+   {
+       text: 'Configuring Atlantis',
+       link: '/your/path'
+       collapsible: true,
+       children: [
+           {
+               text: 'Overview',
+               link: 'configuring-atlantis',
+           },
+  ```
+
+> Note the change in some of the keys used also:  `title -> text` and `path -> link`
+
+
 ### Official Plugins Change
 
 Check the v2 docs of official plugins.