Add a security policy (#3542)

Author: hazzik

SECURITY.md

@@ -0,0 +1,16 @@
+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 5.5.x   | :white_check_mark: |
+| 5.4.x   | :white_check_mark: |
+| < 5.4   | :x:                |
+
+## Reporting a Vulnerability
+
+**Please do NOT report security vulnerabilities through public GitHub issues.**
+
+Instead, please submit vulnerabilities using this [form](https://github.com/nhibernate/nhibernate-core/security/advisories/new). 
+If the issue is confirmed, we will try to release a patch version as soon as possible depending on complexity.