Allow to notify the users about breaking changes and migrations via notice file

In the .NET world it is possible to include `readme.txt` file [that gets open while installing new nuget package](https://docs.microsoft.com/en-us/nuget/create-packages/creating-a-package#add-a-readme-and-other-files). It is used to notify about important breaking changes or migration strategy.

I suspect it would be awesome to have this kind of tool working somewhere between `pub.dev`, `pub get` and IDE extensions. I understand that this wouldn't be feature of the `pub.dev` per se, but I couldn't think of a better place to start a discussion about it.

**How I imagine this could work**

The library maintainer could include a new file (e.g. `./notice/notice-1.2.3.md`) that would be opened whenever the package version changes from any lower to `1.2.3`. The version change could be detected based on previous and current `pubspec.lock` entry. This file would either be printed in the console or opened in IDE.

If there are more files between previous and current version, then only open the most recent one.