Add a guide for running airflow tasks on custom nodes (#871)

airflow/plural/docs/running-on-custom-nodes.md

@@ -0,0 +1,68 @@
+## Running Airflow Tasks on Custom Node Group
+
+In order to point your tasks at a custom node group, you will need to use the `KubernetesExecutor`
+
+There may be a desire to run your Airflow tasks on a specific node size for large workloads, or maybe even 
+[spot instances](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-spot-instances.html) to achieve higher cost 
+savings.
+
+> Disclaimer: if you run your Airflow workloads on spot instances, it is highly recommended to [set retries](https://docs.astronomer.io/learn/rerunning-dags) 
+> for your tasks as they may lose their underlying compute at any time
+
+### create custom node group
+
+In order to run your Airflow Tasks on custom configure nodes, you will need to first follow [these docs](https://docs.plural.sh/operations/cluster-configuration#modifying-node-types)
+to create your desired nodes. For example, if you were on AWS and wanted to use spot instances you would add something 
+like this to your `bootstrap/terraform/main.tf` file:
+
+```yaml
+multi_az_node_groups = {
+  medium_burst_spot = {
+      name = "medium-burst-spot"
+      min_capacity = 3
+      desired_capacity = 3
+      instance_types = ["t3.xlarge", "t3a.xlarge"]
+      capacity_type = "SPOT"
+      k8s_labels = {
+        "plural.sh/capacityType" = "SPOT"
+        "plural.sh/performanceType" = "BURST"
+        "plural.sh/scalingGroup" = "medium-burst-spot"
+      }
+      k8s_taints = [{
+        key = "plural.sh/capacityType"
+        value = "SPOT"
+        effect = "NO_SCHEDULE"
+      }]
+    }
+}
+```
+
+Then run `plural deploy --commit "add more spot nodes"` to update your cluster.
+
+> ! If you get an error like `InvalidParameterException: Minimum capacity 3 can't be greater than desired size 0` you 
+> may have to use your cloud CLI or console to enact the change manually and then try running again.
+
+### update airflow to use node group
+
+After creating your custom node group, you can point configure Airflow to use it by adding the following to your 
+`./airflow/helm/values.yaml` (this can also be done in the plural application console)
+
+```yaml
+airflow:
+  airflow:
+    airflow:
+      config:
+      kubernetesPodTemplate:
+        nodeSelector:
+          plural.sh/capacityType: SPOT
+        tolerations:
+        - effect: NoSchedule
+          key: plural.sh/capacityType
+          operator: Equal
+          value: SPOT
+```
+
+### redeploy
+
+From there, you should be able to run `plural build --only airflow && plural deploy --commit "run on spot instances"` to 
+use the custom node group to execute your tasks.