azure/terraform/README.md: Describe what the vault.tf is about

azure/terraform/README.md

+# What
+
+Terraform allows to transparently declare our infrastructure as code.
+
+
+# The road so far
+
+Only the vault is defined within the `vault.tf` file.
+
+Vault is composed of:
+- one api allowing to request object cooking or retrieve cooked objects
+  (objstorage, db)
+- this also uses a storage to read the swh archive (azure:
+  storage0.euwest.azure)
+
+The vault.tf defines here:
+- existing:
+  - subnet (reuse)
+  - security-group (reuse)
+- new resource:
+  - euwest-vault: to group together the allocated resource for the vault
+  - vangogh-interface: to define an ip for the new server vangogh
+  - vault-storage: storage account for the BlobStorage necessary for the
+    objstorage api of the vault (including a container "contents" to actually
+    store the blobs)
+  - vault-server: the 'vangogh.euwest.azure' vm to actually serve the vault api
+
 # Install terraform
 
 https://learn.hashicorp.com/terraform/getting-started/install.html#installing-terraform
@@ -30,7 +57,8 @@ example the admin user and its associated public key
 
 # Apply changes
 
-Same as previous command except that it applies the diff to the infra:
+Same as previous command except that it applies the diff to the infra
+(interactive):
 
 ```
 terraform apply