List prerequisites (#1133)

Namely support for only x86-64/AMD64.

Author: dan-mckean

docs/install-upgrade.md

@@ -2,6 +2,7 @@
 
 ## Table of Contents
 
+- [Prerequisites](#prerequisites)
 - [Install the Operator](#install-the-operator)
   - [Understand Deployment Scopes](#understand-deployment-scopes)
     - [Operator in Same Namespace as Resources](#operator-in-same-namespace-as-resources)
@@ -17,6 +18,10 @@
 - [Upgrade the Operator](#upgrade-the-operator)
 - [Rotating TLS certificate for the MongoDB deployment](#rotating-tls-certificate-for-the-mongodb-deployment)
 
+## Prerequisites
+
+- A Kubernetes cluster with nodes with x86-64/AMD64 processors (either all, or a separate node pool)
+
 ## Install the Operator
 
 The MongoDB Community Kubernetes Operator is a [Custom Resource Definition](https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/) and a controller.
@@ -293,4 +298,4 @@ kubectl apply -f -
 *`secret_name` is what you've specified under `Spec.Security.TLS.CertificateKeySecret.Name`*.
 
 If you're using a tool like cert-manager, you can follow [these instructions](https://cert-manager.io/docs/usage/certificate/#renewal) to rotate the certificate.
-The operator should would watch the secret change and re-trigger a reconcile process. 
+The operator should would watch the secret change and re-trigger a reconcile process.