chore: add security policy (#310)

Signed-off-by: guillaume <[email protected]>

SECURITY.md

@@ -0,0 +1,7 @@
+# Security policy
+
+To report a security issue, file a [Private Security Report](https://github.com/canonical/vault-operator/security/advisories/new)
+with a description of the issue, the steps you took to create the issue, affected versions, and, if known, mitigations for the issue.
+
+The [Ubuntu Security disclosure and embargo policy](https://ubuntu.com/security/disclosure-policy) contains more information about
+what you can expect when you contact us and what we expect from you.