Create SECURITY.md

Author: Krinkle

SECURITY.md

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+# Security policy
+
+## Supported versions
+
+The [latest release](https://github.com/cssjanus/php-cssjanus/releases) is supported with security updates.
+
+## Reporting a vulnerability
+
+Wikimedia takes security seriously. If you believe you have found a security issue,
+see [mediawiki.org/wiki/Reporting_security_bugs](https://www.mediawiki.org/wiki/Reporting_security_bugs)
+for how to responsibly report it, so we can take steps to address it as quickly as possible.
+
+Thanks!