[SPARK-52117] Add README.md to Helm Chart

Author: dongjoon-hyun

build-tools/helm/spark-kubernetes-operator/README.md

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+
+# Helm Chart for Apache Spark
+
+[![Artifact Hub](https://img.shields.io/endpoint?url=https://artifacthub.io/badge/repository/spark-kubernetes-operator)](https://artifacthub.io/packages/search?repo=spark-kubernetes-operator)
+
+[Apache Spark™](https://spark.apache.org/) is a unified analytics engine for large-scale data processing, and
+Apache Spark™ K8s Operator is a subproject of Apache Spark and
+aims to extend K8s resource manager to manage Apache Spark applications and clusters via
+[Operator Pattern](https://kubernetes.io/docs/concepts/extend-kubernetes/operator/).
+
+## Introduction
+
+This chart will bootstrap an [Apache Spark K8s Operator](https://github.com/apache/spark-kubernetes-operator/) deployment on a [Kubernetes](http://kubernetes.io)
+cluster using the [Helm](https://helm.sh) package manager. With this, you can launch Spark applications and clusters.
+
+## Requirements
+
+- Kubernetes 1.30+ cluster
+- Helm 3.0+
+
+## Features
+
+- Support Apache Spark 3.5 and 4.0 (RC5)
+- Support `SparkApp` and `SparkCluster` CRDs
+  - `sparkapplications.spark.apache.org` (v1alpha1)
+  - `sparkclusters.spark.apache.org` (v1alpha1)
+- Support HPA for SparkCluster