feat: Update documentation for adopting the reusable Terraform workflow

[SamW94]

Description

Update the README to provide instruction on pre-requisites to adopting the reusable Terraform workflow.

Contributors

@SamW94

Type of change

• Refactoring (made code better without changing its behaviour)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How this has been tested

N/A - documentation change only.

Checklist

• I have performed a self-review of my code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Reviewer Checklist

• I have reviewed the PR and ensured no secret values are present

[github-actions]

Dependency Review

✅ No vulnerabilities or OpenSSF Scorecard issues found.

Scanned Files

None