Document HTTP backend and module registry integration

.gitlab/README.md.template

@@ -97,6 +97,34 @@ fmt:
 ...
 ```
 
+### GitLab-managed Terraform state backend
+
+This component - by leveraging the [`gitlab-tofu`](src/gitlab-tofu.sh) CLI internally -
+automatically configures the
+[GitLab-managed Terraform state backend](https://docs.gitlab.com/ee/user/infrastructure/iac/terraform_state.html).
+The only thing required is that the Terraform configuration must specify an empty `http` backend block, like this:
+
+```hcl
+terraform {
+  backend "http" {}
+}
+```
+
+We recommend having a dedicated `backend.tf` file inside your `root_dir`
+with the aforementioned block.
+
+### Access to Terraform Module Registry
+
+Similar to automatically configuring the [GitLab-managed Terraform state backend]
+the component also sets up credentials to authenticate with the
+[Terraform Module Registry](https://docs.gitlab.com/ee/user/packages/terraform_module_registry/)
+of the project the pipeline runs in.
+It basically sets the `TF_TOKEN_<domain>` variable to the `$CI_JOB_TOKEN`, where `<domain>` is
+the GitLab instance domain, for example for GitLab.com this would set `TF_TOKEN_gitlab_com` to
+the `$CI_JOB_TOKEN`. However, it'll only do so if the variable is not already provided.
+Thus, if you want to authenticate differently or to another Terraform Module Registry,
+you may just provide the `TF_TOKEN_<domain>` variable yourself, e.g. via CI/CD variables.
+
 ### Opinionated Templates
 
 This component repository also provides some templates that may often be used,

README.md

@@ -99,6 +99,34 @@ fmt:
 ...
 ```
 
+### GitLab-managed Terraform state backend
+
+This component - by leveraging the [`gitlab-tofu`](src/gitlab-tofu.sh) CLI internally -
+automatically configures the
+[GitLab-managed Terraform state backend](https://docs.gitlab.com/ee/user/infrastructure/iac/terraform_state.html).
+The only thing required is that the Terraform configuration must specify an empty `http` backend block, like this:
+
+```hcl
+terraform {
+  backend "http" {}
+}
+```
+
+We recommend having a dedicated `backend.tf` file inside your `root_dir`
+with the aforementioned block.
+
+### Access to Terraform Module Registry
+
+Similar to automatically configuring the [GitLab-managed Terraform state backend]
+the component also sets up credentials to authenticate with the
+[Terraform Module Registry](https://docs.gitlab.com/ee/user/packages/terraform_module_registry/)
+of the project the pipeline runs in.
+It basically sets the `TF_TOKEN_<domain>` variable to the `$CI_JOB_TOKEN`, where `<domain>` is
+the GitLab instance domain, for example for GitLab.com this would set `TF_TOKEN_gitlab_com` to
+the `$CI_JOB_TOKEN`. However, it'll only do so if the variable is not already provided.
+Thus, if you want to authenticate differently or to another Terraform Module Registry,
+you may just provide the `TF_TOKEN_<domain>` variable yourself, e.g. via CI/CD variables.
+
 ### Opinionated Templates
 
 This component repository also provides some templates that may often be used,