docs: Use dedicated README file for PackerCLI project

[and-hus]

The repository contains a global README.md file, which nicely introduces the complete SDK and provides some generic information and links. Currently all assets/packages from the repository are configured to use this README.md file when packaging. This means, that this very generic README is also shown e.g. on nuget.org for each package (see e.g. HL7.Cql.Packaging).

I would propose to use dedicated README.md files for each package.
This allows to provide more detailed and more specific documentation and information for the respective package.

As an example, this PR adds a dedicated README.md for the PackagerCli project, which builds into the HL7.Cql.Packaging nuget package. The project configuration is modified to use this dedicated README instead of the global README which is configured via the cql-sdk.props file that is imported at the top of the csproj-file.

When adopting this approach for all projects, we might even remove the inclusion of the global README in the cql-sdk.props file and just go with project specific includes. This would avoid the Remove step.

[baseTwo]

We have a ticket for updating the documentation #409 , once continue working on it, I'll also review this PR.

[baseTwo]

Thanks for the contribution