Add security policy (#530)

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+The .NET Framework support policy, including supported versions, can be found at the [.NET Framework Support Policy page](https://dotnet.microsoft.com/platform/support/policy/dotnet-framework).
+
+## Reporting a Vulnerability
+
+Security issues and bugs should be reported privately to the Microsoft Security Response Center (MSRC), either by emailing secure@microsoft.com or via the portal at https://msrc.microsoft.com. You should receive a response within 24 hours. If for some reason you do not, please follow up via email to ensure we received your original message. Further information, including the MSRC PGP key, can be found in the [MSRC Report an Issue FAQ](https://www.microsoft.com/en-us/msrc/faqs-report-an-issue).
+
+Please do not open issues for anything you think might have a security implication.