[Docs][kubectl-plugin] Add doc for install via Krew

Closes: #2457
Signed-off-by: Chi-Sheng Liu <[email protected]>

kubectl-plugin/README.md

@@ -9,6 +9,13 @@ Kubectl plugin/extension for Kuberay CLI that provides the ability to manage ray
 
 ## Installation
 
+### Install using Krew kubectl plugin manager
+
+1. Install [Krew](https://krew.sigs.k8s.io/docs/user-guide/setup/install/).
+2. Download the plugin list by running `kubectl krew update`.
+3. Install the plugin by running `kubectl krew install ray`.
+4. Run `kubectl ray --help` to verify the installation.
+
 ### Download from GitHub releases
 
 Go to the [releases page](https://github.com/ray-project/kuberay/releases) and download the binary for your platform.