mention OpenShift in the installation guide

Author: razvan

docs/modules/zookeeper/pages/getting_started/installation.adoc

@@ -1,7 +1,9 @@
 = Installation
 :description: Install the Stackable operator for Apache ZooKeeper using stackablectl or Helm.
 
-Follow the instructions below to install the Stackable operator for Apache ZooKeeper, using either `stackablectl` or Helm:
+There are multiple ways to install the Stackable Operator for Apache Zookeeper.
+`stackablectl` is the preferred way but Helm is also supported.
+OpenShift users may prefer installing the operator from the RedHat Certified Operator catalog using the OpenShift web console.
 
 [tabs]
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