docs: add security policy

SECURITY.md

@@ -0,0 +1,24 @@
+# Security Policy
+
+- [Security Policy](#security-policy)
+  - [Supported Versions](#supported-versions)
+  - [Reporting a Vulnerability](#reporting-a-vulnerability)
+
+## Supported Versions
+
+Use this section to tell people about which versions of your project are currently being supported
+with security updates.
+
+| Version | Supported          |
+|---------|--------------------|
+| 5.1.x   | :white_check_mark: |
+| 5.0.x   | :x:                |
+| 4.0.x   | :white_check_mark: |
+| \< 4.0  | :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.