Docs: add “HIPAA-safe Quickstart” using synthetic data (no PHI)

[rdlasco]

This PR adds a short “HIPAA-safe Quickstart” section to the README.

• Reminds users not to use real PHI for evaluation.
• Points to synthetic DICOM from the deid-data package.
• Provides a tiny Python snippet that loads one sample file and runs detect.
• Adds safety notes (use synthetic data; verify outputs/logs before sharing).

Scope: documentation only; no behavior change.

Fixes #287