Add readme for EFcore.Desgin NuGet package

[lyndaidaii]

• I've read the guidelines for contributing and seen the walkthrough
• I've posted a comment on an issue with a detailed description of how I am planning to contribute and got approval from a member of the team
• The code builds and tests pass locally (also verified by our automated build checks)
• Commit messages follow this format:

        Summary of the changes
        - Add readme file for nuget package https://www.nuget.org/packages/Microsoft.EntityFrameworkCore.Design/7.0.3

        Fixes #bugnumber

• Tests for the changes have been added (for bug fixes / features)
• Code follows the same patterns and style as existing code in this repo

[lyndaidaii]

This is draft. not ready for review. still working on it.

[bricelam]

See https://stackoverflow.com/a/42584711/475031