Update external shuffle service docs

Author: foxish

resource-managers/kubernetes/architecture-docs/external-shuffle-service.md

@@ -3,4 +3,27 @@ layout: global
 title: Kubernetes Implementation of the External Shuffle Service
 ---
 
+# External Shuffle Service
 
+The `KubernetesExternalShuffleService` was added to allow Spark to use Dynamic Allocation Mode when
+running in Kubernetes. The shuffle service is responsible for persisting shuffle files beyond the
+lifetime of the executors, allowing the number of executors to scale up and down without losing
+computation. 
+
+The implementation of choice is as a DaemonSet that runs a shuffle-service pod on each node.
+Shuffle-service pods and executors pods that land on the same node share disk using hostpath
+volumes. Spark requires that each executor must know the IP address of the shuffle-service pod that
+shares disk with it. 
+
+The user specifies the shuffle service pods they want executors of a particular SparkJob to use
+through two new properties:
+
+* spark.kubernetes.shuffle.service.labels
+* spark.kubernetes.shuffle.namespace
+
+KubernetesClusterSchedulerBackend is aware of shuffle service pods and the node corresponding to
+them in a particular namespace. It uses this data to configure the executor pods to connect with the
+shuffle services that are co-located with them on the same node. 
+
+There is additional logic in the `KubernetesExternalShuffleService` to watch the Kubernetes API,
+detect failures, and proactively cleanup files in those error cases.