📃 Add documentation for cloud credentials

prepare-labs/README.md

@@ -59,6 +59,27 @@ You don't **have to** install the CLI tools of the cloud provider(s) that you wa
 
 If you want to provide your cloud credentials through other means, you will have to adjust the Terraform configuration files in `terraform/provider-config` accordingly.
 
+Here is where we look for credentials for each provider:
+
+- AWS: Terraform defaults; see [AWS provider documentation][creds-aws] (for instance, you can use the `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` environment variables, or AWS config and profile files)
+- Azure: Terraform defaults; see [AzureRM provider documentation][creds-azure] (typically, you can authenticate with the `az` CLI and Terraform will pick it up automatically)
+- Civo: CLI configuration file (`~/.civo.json`)
+- Digital Ocean: CLI configuration file (`~/.config/doctl/config.yaml`)
+- Exoscale: CLI configuration file (`~/.config/exoscale/exoscale.toml`)
+- Google Cloud: FIXME, note that the project name is currently hard-coded to `prepare-tf`
+- Hetzner: CLI configuration file (`~/.config/hcloud/cli.toml`)
+- Linode: CLI configuration file (`~/.config/linode-cli`)
+- OpenStack: you will need to write a tfvars file (check [that exemple](terraform/virtual-machines/tfvars.example))
+- Oracle: Terraform defaults; see [OCI provider documentation][creds-oci] (for instance, you can set up API keys; or you can use a short-lived token generated by the OCI CLI with `oci session authenticate`)
+- OVH: Terraform defaults; see [OVH provider documentation][creds-ovh] (this typically involves setting up 5 `OVH_...` environment variables)
+- Scaleway: Terraform defaults; see [Scaleway provider documentation][creds-scw] (for instance, you can set environment variables, but it will also automatically pick up CLI authentication from `~/.config/scw/config.yaml`)
+
+[creds-aws]: https://registry.terraform.io/providers/hashicorp/aws/latest/docs#authentication-and-configuration
+[creds-azure]: https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs#authenticating-to-azure
+[creds-oci]: https://docs.oracle.com/en-us/iaas/Content/API/SDKDocs/terraformproviderconfiguration.htm#authentication
+[creds-ovh]: https://registry.terraform.io/providers/ovh/ovh/latest/docs#provider-configuration
+[creds-scw]: https://registry.terraform.io/providers/scaleway/scaleway/latest/docs#authentication
+
 ## General Workflow
 
 - fork/clone repo