Add breaking changes tip in docs (cfug#1844)

Resolves cfug#1843.

### New Pull Request Checklist

- [x] I have read the
[Documentation](https://pub.dev/documentation/dio/latest/)
- [x] I have searched for a similar pull request in the
[project](https://github.com/cfug/dio/pulls) and found none
- [x] I have updated this branch with the latest `main` branch to avoid
conflicts (via merge from master or rebase)
- [ ] I have added the required tests to prove the fix/feature I'm
adding
- [x] I have updated the documentation (if necessary)
- [ ] I have run the tests without failures
- [ ] I have updated the `CHANGELOG.md` in the corresponding package

### Additional context and info (if any)

Apparently, the previous tips are not obvious enough.

---------

Signed-off-by: Alex Li <[email protected]>

README-ZH.md

@@ -9,6 +9,9 @@ Language: [English](README.md) | 简体中文
 - dio: [链接](dio)
   [![Pub](https://img.shields.io/pub/v/dio.svg?label=dev&include_prereleases)](https://pub.flutter-io.cn/packages/dio)
 
+**在你更新之前：大版本和次要版本可能会包含不兼容的重大改动。<br/>
+请阅读 [迁移指南](dio/migration_guide.md) 了解完整的重大变更内容。**
+
 ### 插件
 
 - cookie_manager: [链接](plugins/cookie_manager)

README.md

@@ -10,6 +10,9 @@ Please move specific paths for projects instructions.
 - dio: [link](dio)
   [![Pub](https://img.shields.io/pub/v/dio.svg?label=dev&include_prereleases)](https://pub.dev/packages/dio)
 
+**Before you upgrade: Breaking changes might happen in major and minor versions of packages.<br/>
+See the [Migration Guide](dio/migration_guide.md) for the complete breaking changes list.**
+
 ### Plugins
 
 - cookie_manager: [link](plugins/cookie_manager)

dio/CHANGELOG.md

@@ -1,5 +1,8 @@
 # CHANGELOG
 
+**Before you upgrade: Breaking changes might happen in major and minor versions of packages.<br/>
+See the [Migration Guide][] for the complete breaking changes list.**
+
 ## Unreleased
 
 - Make `LogInterceptor` prints in DEBUG mode (when the assertion is enabled) by default.
@@ -324,3 +327,5 @@ First Stable version for 2.x
 ## 0.0.1
 
 - Initial version, created by Stagehand
+
+[Migration Guide]: ./migration_guide.md

dio/README-ZH.md

@@ -52,8 +52,6 @@ dio 是一个强大的 Dart HTTP 请求库，支持全局配置、Restful API、
 
 ## 开始使用
 
-> 查看 [迁移指南](migration_guide.md) 以了解各个版本之间的重大变更。
-
 ### 添加依赖
 
 你可以使用以下命令将 dio 的最新稳定版依赖添加至你的项目：
@@ -72,6 +70,9 @@ dependencies:
 最新稳定版本为：![Pub](https://img.shields.io/pub/v/dio.svg)
 最新包含开发版的版本为：![Pub](https://img.shields.io/pub/v/dio?include_prereleases)
 
+**在你更新之前：大版本和次要版本可能会包含不兼容的重大改动。<br/>
+请阅读 [迁移指南][] 了解完整的重大变更内容。**
+
 ## 一个极简的示例
 
 ```dart
@@ -867,5 +868,6 @@ class MyDio with DioMixin implements Dio {
 
 你可以参考简单请求的定义修改你的请求，或者为你的服务加上 CORS 中间件进行跨域处理。
 
+[迁移指南]: ./migration_guide.md
 [简单请求]: https://developer.mozilla.org/zh-CN/docs/Web/HTTP/CORS#%E7%AE%80%E5%8D%95%E8%AF%B7%E6%B1%82
 [CORS 预检]: https://developer.mozilla.org/zh-CN/docs/Glossary/Preflight_request

dio/README.md

@@ -53,8 +53,6 @@ timeout, and custom adapters etc.
 
 ## Get started
 
-> Checkout the [Migration Guide](migration_guide.md) for breaking changes between versions.
-
 ### Add dependency
 
 You can use the command to add dio as a dependency with the latest stable version:
@@ -73,6 +71,9 @@ dependencies:
 The latest version is: ![Pub](https://img.shields.io/pub/v/dio.svg)
 The latest version including pre-releases is: ![Pub](https://img.shields.io/pub/v/dio?include_prereleases)
 
+**Before you upgrade: Breaking changes might happen in major and minor versions of packages.<br/>
+See the [Migration Guide][] for the complete breaking changes list.**
+
 ### Super simple to use
 
 ```dart
@@ -918,5 +919,6 @@ and a server is aware using specific methods and headers.
 You can modify your requests to match the definition of simple request,
 or add a CORS middleware for your service to handle CORS requests.
 
+[Migration Guide]: ./migration_guide.md
 [simple request]: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#simple_requests
 [CORS preflight request]: https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request