Add Security Policy (#13946)

SECURITY.md

@@ -0,0 +1,12 @@
+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 3.1.x   | :white_check_mark: |
+| < 3.1   | :x:                |
+
+## Reporting a Vulnerability
+
+hello@joinmastodon.org