Add security.md with policy (#822)

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+To report a security issue, please follow the steps below:
+
+Using GitHub, file a [Private Security Report](https://github.com/canonical/k8s-snap/security/advisories/new) with:
+- A description of the issue
+- Steps to reproduce the issue
+- Affected versions of the `k8s-snap` package
+- Any known mitigations for the issue
+
+The [Ubuntu Security disclosure and embargo policy](https://ubuntu.com/security/disclosure-policy) contains more information about what to expect during this process and our requirements for responsible disclosure.
+
+Thank you for contributing to the security and integrity of `k8s-snap`!