Add SECURITY.md

This adds a security disclosure document based on the NHS software
engineering quality framework template.

Author: thomasleese

SECURITY.md

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+# Security
+
+We take security and the protection of private data extremely seriously. If you believe you have found a vulnerability or other issue which has compromised or could compromise the security of any of our systems or private data managed by our systems, please do not hesitate to contact us using the method outlined below.
+
+## Reporting a vulnerability
+
+If you believe you have found a security issue in this repository, please report it using GitHub's private vulnerability reporting:
+
+1. [Report a vulnerability](https://github.com/NHSDigital/manage-vaccinations-in-schools/security/advisories/new)
+1. Provide details of the issue and steps to reproduce
+
+This creates a private channel for discussion and allows us to coordinate a fix before any public disclosure.
+
+## General Security Enquiries
+
+If you have general enquiries regarding our cybersecurity, please reach out to us at [cybersecurity@nhs.net](cybersecurity@nhs.net)