create github security policy

SECURITY.md

@@ -0,0 +1,16 @@
+# Security Policy
+
+## Supported Versions
+
+These borg releases are currently supported with security updates.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 1.1.x   | :white_check_mark: |
+| < 1.1   | :x:                |
+
+## Reporting a Vulnerability
+
+See there:
+
+https://borgbackup.readthedocs.io/en/latest/support.html#security-contact