chore: Create central security policy document

This was copied from syft/SECURITY.md and should apply to all of our repositories.

Signed-off-by: Alan Pope <[email protected]>

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Security updates are applied only to the most recent release, try to always be up to date.
+
+## Reporting a Vulnerability
+
+To report a security issue, please email
+[[email protected]](mailto:[email protected])
+with a description of the issue, the steps you took to create the issue,
+affected versions, and, if known, mitigations for the issue.
+
+All support will be made on the best effort base, so please indicate the "urgency level" of the vulnerability as Critical, High, Medium or Low.