feat(test): update README by terraform-docs

[suzuki-shunsuke]

Close #1859

By default, this feature is disabled.

tfaction-root.yaml:

terraform_docs:
  enabled: true

tfaction.yaml:

terraform_docs:
  enabled: true

terraform-docs configuration

tfaction searches a configuration file:

1. .terraform-docs.ya?ml in service directory
2. .config/.terraform-docs.ya?ml in service directory
3. .terraform-docs.ya?ml in the repository root directory
4. .config/.terraform-docs.ya?ml in the repository root directory

https://terraform-docs.io/user-guide/configuration/

[suzuki-shunsuke]

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