Create SECURITY.md policy + post security audit

[caniszczyk]

Fluentd should create a SECURITY.md file which is the standard way for GitHub now to deal with security disclosures. You should also post a link to the 2019 audit: https://cure53.de/pentest-report_fluent.pdf

Once you do, I recommend updating your README like this:
https://github.com/envoyproxy/envoy#security

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@caniszczyk this issue was automatically closed because it did not follow the issue template

[edsiper]

@caniszczyk is this ok ?

https://github.com/fluent/fluentd#security

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