Add README.md to NuGet packages

[nohwnd]

Summary

Add README.md files to all NuGet packages produced by vstest to resolve the 'missing a readme' warnings during \�uild.cmd -pack.

Closes #15549

Changes

• Created README.md for 12 packages: Microsoft.NET.Test.Sdk, Microsoft.TestPlatform.Build, Microsoft.TestPlatform.ObjectModel, Microsoft.TestPlatform.TestHost, Microsoft.TestPlatform.AdapterUtilities, Microsoft.TestPlatform.TranslationLayer, Microsoft.TestPlatform.Extensions.TrxLogger, Microsoft.CodeCoverage, Microsoft.TestPlatform, Microsoft.TestPlatform.CLI, Microsoft.TestPlatform.Portable, Microsoft.TestPlatform.Internal.Uwp
• Updated all .nuspec files (including sourcebuild variants) with <readme>\ metadata and <file>\ entries referencing the README via \$\
• Updated \�ng/verify-nupkgs.ps1\ expected file counts (+1 per package for the new README.md)

Each README includes a package description, usage information, and links to the main repository documentation.

Verification

• \�uild.cmd -pack\ passes with 0 errors and no 'missing a readme' warnings