Create SECURITY.md

Author: adam-fowler

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Currently we support versions 2.x.x of MQTTNIO. These will receive security updates as and when needed.
+
+## Reporting a Vulnerability
+
+If you believe you have found a security vulnerability in MQTTNIO please do not post this in a public forum, do not create a GitHub Issue. Instead you should email [[email protected]](mailto:[email protected]) with details of the issue.
+
+#### What happens next?
+
+* A member of the team will acknowledge receipt of the report within 5
+  working days. This may include a request for additional
+  information about reproducing the vulnerability.
+* We will privately inform the Swift Server Work Group ([SSWG][sswg]) of the
+  vulnerability within 10 days of the report as per their [security
+  guidelines][sswg-security].
+* Once we have identified a fix we may ask you to validate it. We aim to do this
+  within 30 days, but this may not always be possible.
+* We will decide on a planned release date and let you know when it is.
+* Once the fix has been released we will publish a security advisory on GitHub
+  and the [SSWG][sswg] will announce the vulnerability on the [Swift
+  forums][swift-forums-sec].
+
+[sswg]: https://github.com/swift-server/sswg
+[sswg-security]: https://github.com/swift-server/sswg/blob/main/process/incubation.md#security-best-practices
+[swift-forums-sec]: https://forums.swift.org/c/server/security-updates/