docs: Update README to reflect unmaintained status (#378)

README.md

@@ -18,12 +18,15 @@
     under the License.
 
 -->
-# Official Apache Pulsar Helm Chart
 
-This is the officially supported Helm Chart for installing Apache Pulsar on Kubernetes.
+# Apache Pulsar Helm Chart
+
+This project provides Helm Charts for installing Apache Pulsar on Kubernetes.
 
 Read [Deploying Pulsar on Kubernetes](http://pulsar.apache.org/docs/deploy-kubernetes/) for more details.
 
+> :warning: This helm chart is updated outside of the regular Pulsar release cycle and might lag behind a bit. It only supports basic Kubernetes features now. Currently, it can be used as no more than a template and starting point for a Kubernetes deployment. In many cases, it would require some customizations.
+
 ## Features
 
 This Helm Chart includes all the components of Apache Pulsar for a complete experience.