Add security policy

Add a security policy documenting how to report vulnerabilities.

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+To privately report a security vulnerability, please create a security advisory in the [repository's Security tab](https://github.com/justeattakeaway/JustSaying/security/advisories).
+
+Further details can be found in the [GitHub documentation](https://docs.github.com/code-security/security-advisories/guidance-on-reporting-and-writing/privately-reporting-a-security-vulnerability).