Add a security policy for reporting vulnerabilities

Author: mondeja

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+To report a vulnerability in Python-Markdown, [open a private vulnerability report](https://github.com/Python-Markdown/markdown/security/advisories/new) and you can create a patch on a private fork or, after reporting the problem, our maintainers will fix it as soon as possible.