[BUG]: Create a Security Policy

[joycebrum]

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What version (or hash if on master) of pybind11 are you using?

None

Problem description

Hi again, I'd like to suggest another minor change that might improve the overall security posture of pybind11 😄

A Security Policy is a GitHub standard document (SECURITY.md) that can be seen in the "Security Tab" to instruct users about how to report vulnerability in the safest and most efficient way possible.

It is a Scorecard Recommendation (being a security measure of medium priority) and a Github Recommendation.

Together with this issue I'll submit one suggestion of Security Policy, feel free to edit it directly or ask me for editions until it is in compliance with how pybind11 would best handle vulnerability reports.

Reproducible example code

No response

Is this a regression? Put the last known working version here if it is.

Not a regression