Add initial SECURITY.md policy

Signed-off-by: Paul Mars <[email protected]>

SECURITY.md

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+# Security policy
+
+The following documents the ubuntu-image security policy.
+
+## Supported Versions
+
+We support the last version published in the `latest/stable` channel of the [Snapstore](https://snapcraft.io/ubuntu-image).
+
+Previous major versions (1.X and 2.X) are not supported. 
+
+## Reporting a Vulnerability
+
+To report a security issue, please email [[email protected]](mailto:[email protected]) 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.