add security policy

SECURITY.md

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+# Security Policy
+
+The maintainers of the Docker CLI take security seriously. If you discover
+a security issue, please bring it to their attention right away!
+
+## Reporting a Vulnerability
+
+Please **DO NOT** file a public issue, instead send your report privately
+to [security@docker.com](mailto:security@docker.com).
+
+Reporter(s) can expect a response within 72 hours, acknowledging the issue was
+received.
+
+## Review Process
+
+After receiving the report, an initial triage and technical analysis is
+performed to confirm the report and determine its scope. We may request
+additional information in this stage of the process.
+
+Once a reviewer has confirmed the relevance of the report, a draft security
+advisory will be created on GitHub. The draft advisory will be used to discuss
+the issue with maintainers, the reporter(s), and where applicable, other
+affected parties under embargo.
+
+If the vulnerability is accepted, a timeline for developing a patch, public
+disclosure, and patch release will be determined. If there is an embargo period
+on public disclosure before the patch release, the reporter(s) are expected to
+participate in the discussion of the timeline and abide by agreed upon dates
+for public disclosure.
+
+## Accreditation
+
+Security reports are greatly appreciated and we will publicly thank you,
+although we will keep your name confidential if you request it. We also like to
+send gifts - if you're into swag, make sure to let us know. We do not currently
+offer a paid security bounty program at this time.
+
+## Supported Versions
+
+This project uses long-lived branches to maintain releases, and follows
+the maintenance cycle of the Moby project.
+Refer to [BRANCHES-AND-TAGS.md](https://github.com/moby/moby/blob/master/project/BRANCHES-AND-TAGS.md)
+in the default branch of the moby repository to learn about the current
+maintenance status of each branch.