📝 Add SDK constraints in documents.

Author: AlexV525

README-ZH.md

@@ -35,6 +35,10 @@ Language: [English](README.md) | 中文简体
 
 ## 准备工作 🍭
 
+### 版本限制
+
+Flutter SDK：`>=1.20.0` 。
+
 参考:
 - [wechat_assets_picker#准备工作](https://github.com/fluttercandies/flutter_wechat_assets_picker/blob/master/README-ZH.md#preparing-for-use-)
 - [camera#installation](https://pub.dev/packages/camera#installation)

README.md

@@ -61,6 +61,10 @@ This project follows the [all-contributors](https://github.com/all-contributors/
 
 ## Preparing for use 🍭
 
+### Version constraints
+
+Flutter SDK: `>=1.20.0` .
+
 Reference:
 - [wechat_assets_picker#preparing-for-use](https://github.com/fluttercandies/flutter_wechat_assets_picker#preparing-for-use-)
 - [camera#installation](https://pub.dev/packages/camera#installation)