Create SECURITY.md

Author: afrittoli

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+The CDEvents sdk-go only applies security patches to the latest released branch.
+Pull requests may be accepted by the maintainers for previous releases as well on a case by case basis.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 0.3.2   | :white_check_mark: |
+| < 0.3   | :x:                |
+
+
+## Reporting a Vulnerability
+
+Please report vulnerabilities to cdevents-governance[at]googlegroups.com with a description of the issue, the steps you took to create the issue, affected versions, and, if known, mitigations for the issue.