docs: add a security policy document (#504)

Document the security policy for the Pebble project.

See also
[SEC0026](https://docs.google.com/document/d/1RolsiWCWhM-sVmHFjLt-Mq5yKcgQJ8GLXssGJREjSL0/edit).

SECURITY.md

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+# Security policy
+
+## Supported versions
+
+Security updates will be released for all major versions that have had releases in the last year,
+and for all versions of Pebble that are bundled with [Juju](https://github.com/juju/juju)
+releases that [receive security updates](https://juju.is/docs/juju/roadmap).
+
+## Reporting a vulnerability
+
+Please provide a description of the issue, the steps you took to
+create the issue, affected versions, and, if known, mitigations for
+the issue.
+
+The easiest way to report a security issue is through
+[GitHub's security advisory for this project](https://github.com/canonical/pebble/security/advisories/new). See
+[Privately reporting a security
+vulnerability](https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing/privately-reporting-a-security-vulnerability)
+for instructions on reporting using GitHub's security advisory feature.
+
+The Pebble GitHub admins will be notified of the issue and will work with you
+to determine whether the issue qualifies as a security issue and, if so, in
+which component. We will then figure out a fix, get a CVE
+assigned, and coordinate the release of the fix.
+
+You may also send email to [email protected]. Email may optionally be
+encrypted to OpenPGP key
+[`4072 60F7 616E CE4D 9D12 4627 98E9 740D C345 39E0`](https://keyserver.ubuntu.com/pks/lookup?op=get&search=0x407260f7616ece4d9d12462798e9740dc34539e0)
+
+If you have a deadline for public disclosure, please let us know.
+Our vulnerability management team intends to respond within 3 working
+days of your report. This project aims to resolve all vulnerabilities
+within 90 days.
+
+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.