Add SpotVM doc

docs/topics/spot-vms.md

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+# Spot Virtual Machines
+
+[Azure Spot Virtual Machines](https://azure.microsoft.com/en-gb/pricing/spot/) allow users to reduce the costs of their
+compute resources by utilising Azure's spare capacity for a lower price.
+
+With this lower cost, comes the risk of preemption.
+When capacity within a particular Availability Zone is increased,
+Azure may need to reclaim Spot Virtual Machines to satisfy the demand on their data centres.
+
+## When should I use Spot Virtual Machines?
+
+Spot Virtual Machines are ideal for workloads that can be interrupted.
+For example, short jobs or stateless services that can be rescheduled quickly,
+without data loss, and resume operation with limited degradation to a service.
+
+## How do I use Spot Virtual Machines?
+
+**Note**: This feature is only available on Machines at present and not on the
+experimental MachinePools.
+
+To enable a Machine to be backed by a Spot Virtual Machine, add `spotMarketOptions`
+to your `AzureMachineTemplate`:
+
+```yaml
+apiVersion: exp.infrastructure.cluster.x-k8s.io/v1alpha3
+kind: AzureMachineTemplate
+metadata:
+  name: capz-mp-0
+spec:
+  location: westus2
+  template:
+    osDisk:
+      diskSizeGB: 30
+      managedDisk:
+        storageAccountType: Premium_LRS
+      osType: Linux
+    sshPublicKey: ${YOUR_SSH_PUB_KEY}
+    vmSize: Standard_D2s_v3
+    spotMarketOptions: {}
+```
+
+You may also add a `maxPrice` to the options to limit the maximum spend for the
+instance. It is however, recommend **not** to set a `maxPrice` as Azure will
+cap your spending at the on-demand price if this field is left empty and you will
+experience fewer interruptions.
+
+```yaml
+spec:
+  template:
+    spotMarketOptions:
+      maxPrice: "0.04" Price in USD per hour (up to 5 decimal places)
+```