github: Add SECURITY.md for how to report security issues

Signed-off-by: Simon Deziel <simon.deziel@canonical.com>

Author: simondeziel

SECURITY.md

@@ -0,0 +1,9 @@
+To report a security issue, file a [Private Security
+Report](https://github.com/canonical/microcloud/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.