Include root README.md in all NuGet packages

[Copilot]

NuGet packages lacked a readme, leaving users without getting-started information on the package page.

Changes

• Directory.Build.props: Added <PackageReadmeFile>README.md</PackageReadmeFile> and a global ItemGroup that packs the repo root README.md into all NuGet packages via $(MSBuildThisFileDirectory)README.md

<PackageReadmeFile>README.md</PackageReadmeFile>
...
<ItemGroup>
    <None Include="$(MSBuildThisFileDirectory)README.md" Pack="true" PackagePath="\" />
</ItemGroup>

Applies to all SDK-style projects (NJsonSchema, NJsonSchema.Annotations, NJsonSchema.Yaml, NJsonSchema.CodeGeneration, NJsonSchema.CodeGeneration.CSharp, NJsonSchema.CodeGeneration.TypeScript) without any per-project changes.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Improve base package readme</issue_title>
<issue_description>Is your feature request related to a problem? Please describe.
The current package readme is not informative about how to use the package

Describe the solution you'd like

The nuget package should be using the same readme as the repo to make it as easy as possible for a user to get started with the package.

Describe alternatives you've considered
Accept needing to go to github to get information

Additional context
N/a</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve base package readme #1688

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