Create a `SECURITY.md`

[aiman]

There are already instructions on how to report security vulnerabilities:
https://github.com/autonity/autonity.py?tab=readme-ov-file#reporting-a-vulnerability

However it may be better to use a dedicated SECURITY.md, which has Github integration: https://docs.github.com/en/code-security/getting-started/adding-a-security-policy-to-your-repository.