Add a security policy

SECURITY.md

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+# Security Policy
+
+## Reporting a vulnerability
+
+Please report security issues privately rather than in a public issue.
+
+Use GitHub's private vulnerability reporting: the **Report a vulnerability** button on
+the [Security tab](https://github.com/innolitics/dicom-standard/security/advisories/new).
+That opens a thread visible only to the maintainers and the reporter.
+
+Please don't test against the live site. If a proof of concept would change server state,
+describe it rather than running it, or reproduce it locally. The published bundles ship
+source maps, so the browser can be checked offline.
+
+## Scope
+
+This repository publishes the DICOM Standard as JSON, and that data backs the DICOM
+Standard Browser at <https://dicom.innolitics.com>. Reports for either belong here:
+
+- **This repository**: the Python parsing code and the generated `standard/*.json`.
+- **The hosted browser**, including the File Editor, which already sends its bug reports
+  to this tracker.
+
+Out of scope: values that are parsed faithfully from the Standard but look wrong. Those
+are ordinary issues, and a defect in the Standard itself needs a Correction Proposal to
+NEMA.
+
+## What to include
+
+Where the problem is, how to reproduce it, and what it would let someone do. A minimal
+reproduction is more useful than a long report.