docs: Update README for PodDisruptionBudget issue during Pulsar upgra…

…de (#420)

README.md

@@ -238,6 +238,37 @@ Caused by: org.rocksdb.RocksDBException: while open a file for lock: /pulsar/dat
     ... 13 more
 ```
 
+### Recovering from `helm upgrade` error "unable to build kubernetes objects from current release manifest"
+
+Example of the error message:
+```bash
+Error: UPGRADE FAILED: unable to build kubernetes objects from current release manifest:
+[resource mapping not found for name: "pulsar-bookie" namespace: "pulsar" from "":
+no matches for kind "PodDisruptionBudget" in version "policy/v1beta1" ensure CRDs are installed first,
+resource mapping not found for name: "pulsar-broker" namespace: "pulsar" from "":
+no matches for kind "PodDisruptionBudget" in version "policy/v1beta1" ensure CRDs are installed first,
+resource mapping not found for name: "pulsar-zookeeper" namespace: "pulsar" from "":
+no matches for kind "PodDisruptionBudget" in version "policy/v1beta1" ensure CRDs are installed first]
+```
+
+Helm documentation [explains issues with managing releases deployed using outdated APIs](https://helm.sh/docs/topics/kubernetes_apis/#helm-users) when the Kubernetes cluster has been upgraded
+to a version where these APIs are removed. This happens regardless of whether the chart in the upgrade includes supported API versions.
+In this case, you can use the following workaround:
+
+1. Install the [Helm mapkubeapis plugin](https://github.com/helm/helm-mapkubeapis):
+
+    ```bash
+    helm plugin install https://github.com/helm/helm-mapkubeapis
+    ```
+
+2. Run the `helm mapkubeapis` command with the appropriate namespace and release name. In this example, we use the namespace "pulsar" and release name "pulsar":
+
+    ```bash
+    helm mapkubeapis --namespace pulsar pulsar
+    ```
+
+This workaround addresses the issue by updating in-place Helm release metadata that contains deprecated or removed Kubernetes APIs to a new instance with supported Kubernetes APIs and should allow for a successful Helm upgrade.
+
 ## Uninstall
 
 To uninstall the Pulsar Chart, run the following command: