add readme for infra

infra/README.md

@@ -0,0 +1,21 @@
+# Infra
+
+## Secrets
+
+Provide secrets when asked by the command-line, or save them in a `terraform.tfvars` file:
+
+``` hcl
+gandi_token = XXX
+hcloud_token = YYY
+```
+
+## Importing
+
+To import already existent resources, use the `import` command:
+
+```
+tofu import hcloud_server.crocus_server XXX
+tofu import hcloud_firewall.hcloud_firewall YYY
+```
+
+For Hetzner Cloud, the resource IDs can be found in the URL of the admin console.