Update AWS security group documentation to improve security guidance

[adharsh277]

Update AWS security group documentation to improve security guidance

This PR enhances the README by adding best practices for configuring AWS security groups when provisioning EC2 instances with Terraform. It focuses on restricting SSH access, limiting public exposure, using variables for IP management, and encouraging regular audits of security groups. These changes provide practical guidance to users, even if the Terraform files are not present in the repository.

The update also references GitHub Issue #1848
to help maintainers and users understand the context of these recommendations.

How to use

Reviewers can validate this PR by checking the README changes for clear, actionable guidance on AWS security groups and SSH restrictions. No Terraform files are required to verify the documentation edits.

Testing done

Verified that the README markdown renders correctly on GitHub

Ensured all best practice points are clear and concise

No commands were run since this change is documentation-only

Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)

Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.

[tormath1]

Hi @adharsh277, thanks for your contribution but this not the right repository. This topic has already been discussed in the original issue and there is even a PR: flatcar/flatcar-terraform#36

[adharsh277]

Thank you for the clarification, @tormath1 . I understand now that this documentation update belongs in the flatcar-terraform repository instead of here. I’ll review PR flatcar/flatcar-terraform#36
to check what’s already covered and then contribute any additional improvements there if needed.