Add consistent SECURITY.md (#349)

SECURITY.md

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+# Knative Security Policy
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+We're extremely grateful for security researchers and users that report vulnerabilities to the Knative Open Source Community. All reports are thoroughly investigated by a set of community volunteers.
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+To make a report, please email the private security@knative.team list with the security details and the details expected for all Knative bug reports.
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+See [Knative Security and Disclosure Information](https://knative.dev/docs/reference/security/) for more details.