add readinessGate doc into website and migration guide (kubernetes-sigs#1564)

Author: M00nF1sh

docs/guide/upgrade/migrate_v1_v2.md

@@ -51,6 +51,13 @@ It supports existing AWS resources provisioned by AWSALBIngressController(>=v1.1
         |--------|----------|----------|---------------------------|----------------------|
         |All TCP |TCP       |0 - 65535 |sg-008c920b1(managed LB SG)|elbv2.k8s.aws/targetGroupBinding=shared|                     |
 
+4. If you have used podReadinessGate feature, please refer [PodReadinessGate](../controller/pod_readiness_gate.md) for the guide about new readinessGate configuration. 
+
+    !!!tip "old pod readinessGate"
+        once configured properly, AWS Load Balancer Controller will automatically inject the new format of podReadinessGates into your pods, and remove old podReadinessGates if any.
+        
+        However, we still recommend you to remove the old podReadinessGates from your Deployments since it's not used.
+
 ## Upgrade steps
 1. Determine existing installed AWSALBIngressController version.
 ```console

mkdocs.yml

@@ -10,6 +10,7 @@ nav:
           - Installation: guide/controller/installation.md
           - Configurations: guide/controller/configurations.md
           - Subnet Discovery: guide/controller/subnet_discovery.md
+          - Pod Readiness Gate: guide/controller/pod_readiness_gate.md
       - Ingress:
           - Annotations: guide/ingress/annotations.md
           - Spec: guide/ingress/spec.md