Add docs how to import repositories

README.md

@@ -23,6 +23,16 @@ In order to add new repositories, so that the respective permissions / protectio
           ]
         }
 
+Terraform will then automatically create this repository with the appropriate settings. If you created the repository manually on GitHub before, you need to manually import that repository into Terraform. Therefore, [run the following Terraform command locally](#executing-terraform-actions-locally):
+
+For source repositories:
+
+    terraform import github_repository.repositories[\"your-repository-name\"] your-repository-name
+
+For mirror repositories:
+
+    terraform import github_repository.mirrors[\"your-mirror-repository-name\"] your-mirror-repository-name
+
 ## Changing Terraform Configuration
 
 The process for changing Terraform configuration is as follows:
@@ -31,3 +41,13 @@ The process for changing Terraform configuration is as follows:
 2. GitHub Actions will automatically be run to check the syntax of the updated configuration. For first-time users, an administrator needs to approve the run of this GitHub Action once.
 3. Administrators can now add a comment to the pull request with the text "/plan". This will dry-run the configuration changes and show the output of the applied changes.
 4. When the merge checks are fulfilled, the pull request can be merged. Another GitHub Action will automatically be triggered and will actually apply the changes.
+
+## Executing Terraform Actions Locally
+
+In general, we run Terraform via GitHub Actions. However, in certain cases, it might be handy to execute Terraform locally. Therefore, install Terraform and set the following environment variables:
+
+* `TF_VAR_github_app_id`: The ID of the Mage-OS Terraform GitHub App, which is shown at https://github.com/organizations/mage-os/settings/apps/mage-os-terraform.
+* `TF_VAR_github_app_installation_id`: The installation ID of the Mage-OS Terraform GitHub App, which you find in the URL when you click on "Configure" for the respective app on https://github.com/organizations/mage-os/settings/installations.
+* `TF_VAR_app_node_id`: The node ID of the app, which you find out as described in the [installation docs](INSTALL.md#getting-github-app-node-id).
+* `TF_VAR_github_app_pem_file`: The contents of the private key that you can create on https://github.com/organizations/mage-os/settings/apps/mage-os-terraform.
+* `GOOGLE_APPLICATION_CREDENTIALS`: The (preferably absolute) path to the key file you can generate in Google Cloud > IAM & Admin > Service Accounts > Mage-OS Terraform account > Keys.