v0.5.8: README refresh

What's new

Documentation-only release — README now covers all v0.5.x features:

• --format json output mode with piping examples
• Experiment Tracking (WandB / MLflow)
• Adaptive Sampling (--adaptive)
• Failure Diagnostics (automatic root cause analysis)
• Feature Summary table (16 features with version history)
• Reorganized CI/Pipeline Integration section

Full changelog: https://github.com/maruyamakoju/deltatau-audit/blob/main/CHANGELOG.md

Full Changelog: v0.5.7...v0.5.8