📝 Update migration guide

Author: AlexV525

guides/migration_guide.md

@@ -1,33 +1,80 @@
 # Migration Guide
 
-## Migrate to 5.0.0
+This document gathered all breaking changes and migrations requirement between major versions.
+
+## References
+
+API documentation:
+- `AssetPickerBuilderDelegate`:
+  - https://pub.dev/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerBuilderDelegate-class.html
+  - https://pub.flutter-io.cn/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerBuilderDelegate-class.html
+- `AssetPickerViewerBuilderDelegate`:
+  - https://pub.dev/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerViewerBuilderDelegate-class.html
+  - https://pub.flutter-io.cn/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerViewerBuilderDelegate-class.html
+- `AssetPickerProvider`:
+  - https://pub.dev/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerProvider-class.html
+  - https://pub.flutter-io.cn/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerProvider-class.html
+- `AssetPickerViewerProvider`:
+  - https://pub.dev/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerViewerProvider-class.html
+  - https://pub.flutter-io.cn/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerViewerProvider-class.html
+- `AssetsPickerTextDelegate`:
+  - https://pub.dev/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetsPickerTextDelegate-class.html
+  - https://pub.flutter-io.cn/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetsPickerTextDelegate-class.html
+
+## Major versions
+
+- [6.0.0](#6.0.0)
+- [5.0.0](#5.0.0)
+
+## 6.0.0
+
+### Summary
+
+_If you didn't extend `AssetPickerBuilderDelegate` or `AssetTextDelegate` to build delegates on your own,
+you can stop reading._
+
+- User who extended `AssetPickerBuilderDelegate` needs to update the subclass with the latest changes.
+- `AssetsPickerTextDelegate` is not abstract anymore.
+
+### Details
+
+#### `AssetPickerBuilderDelegate`
+
+- This delegate requires a new argument `initialPermission` with `PermissionState` type when using.
+  The intention of this change is to be capable with various of `PermissionState`.
+  If your delegate didn't require a permission check, you can pass `PermissionState.authorized` directly.
+
+- `assetsGridBuilder` is not implemented by default.
+
+- The `findChildIndexBuilder` and `assetsGridItemCount` methods have new signature.
+  They require calculating placeholders count on iOS/macOS by default.
+
+#### `AssetsPickerTextDelegate`
+
+- This delegate is now a normal class with Chinese language implemented by default,
+  which makes `DefaultAssetsPickerTextDelegate` removed. So if you used to use `DefaultAssetsPickerTextDelegate()`,
+  use `AssetsPickerTextDelegate()` instead.
+
+## 5.0.0
 
 ### Summary
 
 _If you only use the `AssetPicker.pickAssets` and `AssetEntityImageProvider`,
 didn't use `AssetPickerViewer`, `AssetPickerProvider`, or other components separately,
 you can stop reading._
 
-`AssetPicker` and `AssetPickerViewer` are only a builder since 5.x, all widgets construct were moved
-to `AssetPickerBuilderDelegate` and `AssetPickerViewerBuilderDelegate`, and these delegates are both
-abstract.
+The `AssetPicker` and the `AssetPickerViewer` are only a builder since 5.x,
+all widgets construct were moved to `AssetPickerBuilderDelegate` and `AssetPickerViewerBuilderDelegate`,
+and these delegates are both abstract.
 
 By splitting delegates, now you can build your own picker with custom types, style, and widgets.
 
-### Migration steps
+#### Details
 
 For how to implement a custom picker, see the example's custom page for more implementation details.
 
-* If you have ever use `AssetPickerViewer.pushToViewer`, the properties `assets` has changed to
+- If you have ever use `AssetPickerViewer.pushToViewer`, the properties `assets` has changed to
   `previewAssets`.
 
-* If you have extends an `AssetPickerProvider` or `AssetPickerViewerProvider`, it now requires you
+- If you have extended an `AssetPickerProvider` or `AssetPickerViewerProvider`, it now requires you
   to pass generic type `A` and `P`, and handle the entities on your own.
-
-### References
-
-API documentation:
-* `AssetPickerBuilderDelegate`: https://pub.dev/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerBuilderDelegate-class.html
-* `AssetPickerViewerBuilderDelegate`: https://pub.dev/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerViewerBuilderDelegate-class.html
-* `AssetPickerProvider`: https://pub.dev/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerProvider-class.html
-* `AssetPickerViewerProvider`: https://pub.dev/documentation/wechat_assets_picker/latest/wechat_assets_picker/AssetPickerViewerProvider-class.html