Add readme for EF core 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/

        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

Rendered README: https://github.com/dotnet/efcore/blob/32e73aebc1d64bbd519aaeb788547829ff173ecb/src/EFCore/README.md

Address: #30467, #26556

[ajcvickers]

Looks good to me. Just a couple of minor comments.

[ajcvickers]

Superseded by #30934. Thanks @lyndaidaii for doing the work here,