Create SECURITY.md

Author: keradus

.github/SECURITY.md

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+# Security Policy
+
+To report a security issue, please use the GitHub Security Advisory ["Report a Vulnerability"](https://github.com/PHP-CS-Fixer/diff/security/advisories/new) tab.
+
+Report security bugs in third-party libraries to the person or team maintaining that library.