docs: prefer Terraform for infra management (#574)

* docs: prefer Terraform for infra

* Add decision record link

* Rewording

* Update decision record filename

* Link to "use Terraform" decision record

CONTRIBUTING.md

@@ -69,6 +69,8 @@ Minimizing developer follow-up action is [helpful](#positive-helpful-feedback)!
 
 If no Google Cloud resources are needed, use [GitHub Actions](https://docs.github.com/en/actions) to drive automation. Otherwise use Cloud Build ([decision](docs/decisions/2021-05-static-analysis.md)).
 
+Infrastructure (including test fixtures) should be managed automatically. This should be by Terraform where possible, and by custom scripts where necessary. ([decision](docs/decisions/2021-04-terraform.md)) However, some cases exist where resources **cannot** be managed automatically. In those cases, Emblem allows for manual resource management. ([decision](docs/decisions/2022-07-test-fixtures.md))
+
 ## Automated Testing & Productivity
 
 The following automated checks are run against every Pull Request: