feat: add security policy inline with python.org page and similar repos

[JacobCoffee]

What

• Sources a security policy so that users (hopefully) do not create issues on this repo

Why

• There isn't one
• Clear communication for users to see how to report potential security issues

[Mariatta]

Thank you! Can we add this at the org level instead of at the repo level?

[JacobCoffee]

@Mariatta certainly.

https://github.com/python/.github/pull/7 if you want to check that PR.