Add a security policy (#278)

Author: mondeja

SECURITY.md

@@ -0,0 +1,5 @@
+# Security Policy
+
+## Reporting a Vulnerability
+
+To report a vulnerability in mkdocs-include-markdown-plugin, [open a private vulnerability report](https://github.com/mondeja/mkdocs-include-markdown-plugin/security/advisories/new) and you can create a patch on a private fork or, after reporting the problem, the maintainer will fix it as soon as possible.