docs: describe the security policy (#242)

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+<!--
+Use this section to tell people about which versions of your project are
+currently being supported with security updates.
+-->
+
+Only the latest released version currently benefits from security updates. 
+
+| Version  | Supported          |
+| -------- | ------------------ |
+| [latest] | :white_check_mark: |
+| others   | :x:                |
+
+## Reporting a Vulnerability
+
+<!--
+Use this section to tell people how to report a vulnerability.
+
+Tell them where to go, how often they can expect to get an update on a
+reported vulnerability, what to expect if the vulnerability is accepted or
+declined, etc.
+-->
+
+To report a security vulnerability, please open a [draft security advisory] on GitHub.
+
+[latest]: https://github.com/maxGraph/maxGraph/releases/latest
+[draft security advisory]: https://github.com/maxGraph/maxGraph/security/advisories/new