AzureTerraform: README for ARMStrong testing files

[gerrytan]

Adds a README file to clarify the purpose of *.tf files on the ARMStrong testing folder.

ARM (Control Plane) API Specification Update Pull Request

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Generated ApiView

Language	Package Name	ApiView Link
Go	sdk/resourcemanager/terraform/armterraform	https://apiview.dev/Assemblies/Review/503eaf6a29f042d992b397d076b83d9d?revisionId=b5c499fdb475423889595227f341eb40
Java	azure-resourcemanager-terraform	https://apiview.dev/Assemblies/Review/08da9e2c15c743569b06127f7673f6e0?revisionId=9c236f2879724397bca0cd73eb26cf1d

[ramoka178]

Please fix Swagger Avocado. Does not look to be valid format for a readme file

[gerrytan]

Please fix Swagger Avocado. Does not look to be valid format for a readme file

Hi @ramoka178, the README file I added is not meant to be an autorest markdown. It is added because many people thought the directory specification/terraform/resource-manager/Microsoft.AzureTerraform/preview/2023-07-01-preview/terraform/ is a mistake (terraform path appears twice), but it's not: the first is referring to the type of the service (Microsoft.AzureTerraform), and the second is referring to the terraform files used for ARMStrong test (a kind of test we use to ensure service API is conformant to the published spec).

If there is no way around this Swagger Avocado rule then unfortunately I had to decline this PR.

http://azure.github.io/autorest/user/literate-file-formats/configuration.html#the-file-format