Add security policy, such as it is.

docs/SECURITY.md

@@ -0,0 +1,19 @@
+---
+title: Security policy
+permalink: security-policy/index.html
+---
+
+## Supported versions
+
+While we want to hear about security vulnerabilities in all versions of
+borgmatic, security fixes will only be made to the most recently released
+version. It's not practical for our small volunteer effort to maintain
+multiple different release branches and put out separate security patches for
+each.
+
+## Reporting a vulnerability
+
+If you find a security vulnerability, please [file a
+ticket](https://torsion.org/borgmatic/#issues) or [send email
+directly](mailto:witten@torsion.org) as appropriate. You should expect to hear
+back within a few days at most, and generally sooner.