Create SECURITY.md

Author: simonrob

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Using the [latest release of the proxy](https://github.com/simonrob/email-oauth2-proxy/releases/latest) is always recommended.
+Any security issues found in prior versions should be reported, but may only be fixed if they affect the latest version.
+
+## Reporting a Vulnerability
+
+Report vulnerabilities or security issues using [GitHub's security tools](https://github.com/simonrob/email-oauth2-proxy/security).