add terraform intro

Operational_Excellence_deployment.md

@@ -20,4 +20,24 @@ Table of Contents
 
 ## Terraform
 
-# Helm charts
+Terraform provides first-class support to define your resources in Azure in a
+declarative way. The configuration language is concise and very well adapted to
+pull-request-based work models where any infrastructure change is first
+reviewed before being deployed.
+
+See here for the documentation on Azure resources that can be managed by Terraform:
+<https://www.terraform.io/docs/providers/azurerm/>
+
+We recommend setting up only the Azure resources with Terraform, and not what
+runs "inside" them. Think of it really as preparing the infrastructure for your
+workloads. For example, after you have created your VMs with Terraform, you
+should configure operating-system settings with a tool such as Ansible. Also
+for Kubernetes/AKS, we recommend using something like Helm charts / Kustomize,
+and not Terraform.
+
+You should store the Terraform state in Azure storage, so that the state is
+near to the managed resources. This is particularly important if multiple
+persons run Terraform, so that you have proper locking and accounting of the
+resources.
+
+# Helm charts