Add README support

[emgarten]

• Update NuGet.Test.Helpers to get support for readme files in test packages
• README files (if they exist) will now be copied to the flatcontainer folder for that package version along side the icon, nuspec, and nupkg
  • readme files have a content type of text/markdown
  • Files are NOT compressed, this doesn't work with Visual Studio in my testing
• This feature is always on, there isn't a good reason to disable this or make it optional
• The service index json file will now have an entry added for the template to build the README url
  • Existing feeds will need to do a recreate to update the index or may have to edit it manually to get Visual Studio to pick it up

Fixes #212

[Copilot]

Pull Request Overview

This pull request adds support for README files in the NuGet test packages by updating the file copy logic, tests, service index entries, and file system content types.

• Updated tests to include assertions for the README file.
• Extended FlatContainer and file system services to handle README files.
• Updated the service index to include a README URI template.

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/SleetLib.Tests/InitCommandTests.cs	Added a test assertion to verify the README URI template is present.
test/SleetLib.Tests/AddRemoveTests.cs	Introduced a new test for adding and removing packages with a README.
test/Sleet.Azure.Tests/BasicTests.cs	Modified test package setup to include README support.
test/Sleet.AmazonS3.Tests/BasicTests.cs	Modified test package setup to include README support.
src/SleetLib/Services/FlatContainer.cs	Added methods to get and delete the README, and to add the README file.
src/SleetLib/FileSystem/AzureFile.cs	Updated content type handling to recognize README files.
src/SleetLib/FileSystem/AmazonS3File.cs	Updated content type handling to recognize README files.
src/SleetLib/Commands/InitCommand.cs	Added a service index entry for the README URI template and minor refactoring.
build/config.props	Updated NuGetTestHelpersVersion.
ReleaseNotes.md	Added release notes describing the README support feature.