docs: Add AWS Security Group best practices for Terraform users

[adharsh277]

docs: Add AWS Security Group best practices for Terraform users

This PR enhances the flatcar-terraform/README.md by adding AWS Security Group best practices for Terraform users.
It complements #36 and flatcar/Flatcar#1848 by focusing on security improvements not fully covered earlier.

Changes introduced:

• Restricting SSH access to trusted IPs only (avoid 0.0.0.0/0)
• Using variables for IP management (to simplify secure updates)
• Limiting public exposure of ports (only open what is required)
• Encouraging regular audits of security group rules
• Adding notes for handling temporary exceptions securely

These guidelines provide actionable steps for reducing the attack surface when deploying Flatcar instances on AWS with Terraform.

How to use

Reviewers can validate this PR by:

• Checking the updated README.md section on AWS Security Group best practices.
• Verifying that the markdown renders correctly and provides clear, actionable guidance.
• Ensuring recommendations align with standard cloud security practices.

Testing done

• Verified that the README.md renders correctly in GitHub.

• Reviewed text for clarity and accuracy.

• No code execution was required, as this is a documentation-only change.

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

• Inspected CI output for doc-only changes (no binaries, kernel modules, or configs impacted)

Signed-off-by: Adharsh.U 114822057+adharsh277@users.noreply.github.com