add a default security policy

I know we have this page in https://iterative.ai/security-and-privacy, but it'd be nice to have a security policy in GitHub itself. Users will see a link when creating issues in the repositories.

See https://docs.github.com/en/code-security/getting-started/adding-a-security-policy-to-your-repository.
and, https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/creating-a-default-community-health-file#creating-a-repository-for-default-files.