Add a security policy (#327)

Author: ap--

SECURITY.md

@@ -0,0 +1,8 @@
+# Security Policy - Vulnerability Reporting
+
+If you believe you have discovered a security issue in universal-pathlib, do not open a public issue.
+
+Instead, report it via the repository’s **`Security`** tab using the **`Report a vulnerability`** button.
+Include clear details and verify whether the vulnerability is in `universal-pathlib` or one of its dependencies.
+
+Providing a minimal reproducible example will help resolve the issue more efficiently.