Update MetalLB README.md (#6350)

Recently MetalLB becomes one of addons with renaming the options.
This updates MetalLB README.md for this change.

roles/kubernetes-apps/metallb/README.md

@@ -3,17 +3,13 @@
 MetalLB hooks into your Kubernetes cluster, and provides a network load-balancer implementation.
 In short, it allows you to create Kubernetes services of type "LoadBalancer" in clusters that
 don't run on a cloud provider, and thus cannot simply hook into paid products to provide load-balancers.
-This playbook aims to automate [this](https://metallb.universe.tf/concepts/layer2/).
+This addon aims to automate [this](https://metallb.universe.tf/concepts/layer2/).
 It deploys MetalLB into Kubernetes and sets up a layer 2 load-balancer.
 
 ## Install
 
-Defaults can be found in contrib/metallb/roles/provision/defaults/main.yml.
+In the default, MetalLB is not deployed into your Kubernetes cluster.
 You can override the defaults by copying the contents of this file to somewhere in inventory/mycluster/group_vars
-such as inventory/mycluster/groups_vars/k8s-cluster/addons.yml and making any adjustments as required.
-MetalLB allocates external IP addresses from this ip_range option, so you need to update this ip_range option
-at least for suiting your network environment.
-
-```
-ansible-playbook --ask-become -i inventory/sample/hosts.ini contrib/metallb/metallb.yml
-```
+such as inventory/mycluster/groups_vars/k8s-cluster/addons.yml and updating metallb_enabled option to `true`.
+In addition you need to update metallb_ip_range option on the addons.yml at least for suiting your network
+environment, because MetalLB allocates external IP addresses from this metallb_ip_range option.