[SPARK-49397] Add Clean Up section to README.md

### What changes were proposed in this pull request?

This PR aims to document `Clean Up` steps in `README.md` .

### Why are the changes needed?

It's helpful for users to provide clean-up steps explicitly because this is always used during trouble-shooting.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Manual review.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#106 from dongjoon-hyun/SPARK-49397.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

README.md

@@ -117,6 +117,33 @@ $ helm install spark-kubernetes-operator \
 https://nightlies.apache.org/spark/charts/spark-kubernetes-operator-0.1.0-SNAPSHOT.tgz
 ```
 
+## Clean Up
+
+Check the existing Spark applications and clusters. If exists, delete them.
+
+```
+$ kubectl get sparkapp
+No resources found in default namespace.
+
+$ kubectl get sparkcluster
+No resources found in default namespace.
+```
+
+Remove HelmChart and CRDs.
+
+```
+$ helm uninstall spark-kubernetes-operator
+
+$ kubectl delete crd sparkapplications.spark.apache.org
+
+$ kubectl delete crd sparkclusters.spark.apache.org
+```
+
+In case of nightly builds, remove the snapshot image.
+```
+$ docker rmi apache/spark-kubernetes-operator:main-snapshot
+```
+
 ## Contributing
 
 Please review the [Contribution to Spark guide](https://spark.apache.org/contributing.html)