fix: include README.md in published NuGet tool packages

[GarrettBeatty]

Fixes #299

Root cause

The tool packages (Amazon.Lambda.Tools, Amazon.ECS.Tools, Amazon.ElasticBeanstalk.Tools) were published to NuGet.org without a README. The shared MSBuild config buildtools/common.props packed LICENSE and icon.png but never declared or packed a PackageReadmeFile, so NuGet showed no package documentation.

Fix

In buildtools/common.props:

• Add <PackageReadmeFile>README.md</PackageReadmeFile>.
• Add a <None Include=".../README.md" Pack="true" PackagePath="" /> entry so the repo-root README.md is packed into each .nupkg.

Because common.props is shared, this fixes all three tool packages at once.

Versioning

Includes 3 AutoVer Patch change files (one per affected package), each with the changelog message "Include README.md in the published NuGet package."

Reviewer notes

• No unit tests: This is a build/packaging config change with no runtime code path; correctness is verified by the produced .nupkg contents. No automated test guards regression here.