Document build.ps1 usage and pack guard behavior

[atifaziz]

Objective

Update project documentation with the new multi-Roslyn build/test/pack workflow and prerequisite tooling.

Scope

• Add concise usage examples for Build, Test, and Pack flows.
• Explain guard behavior for direct dotnet pack without Roslyn 4.4 artifacts.
• Include cross-platform examples using dotnet pwsh.

Files

• README.md (or CONTRIBUTING.md if maintainers prefer)

Tasks

• Add prerequisite section for dotnet tool restore.
• Add command examples:
  • dotnet pwsh ./build.ps1
  • dotnet pwsh ./build.ps1 -Test
  • dotnet pwsh ./build.ps1 -Pack
  • Optional -VersionSuffix
• Document that plain dotnet pack src/DocoptNet/DocoptNet.csproj is expected to fail unless Roslyn 4.4 artifacts exist.
• Keep docs aligned with actual script parameter names and behavior.

Acceptance Criteria

• Documentation is accurate against implemented script and CI behavior.
• New contributor can run Build/Test/Pack end-to-end from docs alone.
• Doc update is completed after implementation/verification issues are done.

Dependencies

• Add RoslynVersion configuration in project #207
• Add build.ps1 for multi-Roslyn Build/Test/Pack orchestration #208
• Wire CI workflow to build.ps1 across OS matrix #209
• Validate packaging layout and Roslyn variant contents #210

Notes

• This should be completed as the final independent step after technical validation.

[atifaziz]

Closed via PR #214.