docs(eks): add documentation about add-ons (#30815)

### Issue # (if applicable)

None

### Reason for this change

Addon L2 construct is added by #30576 but there is no documentation about it in the README.md.

### Description of changes

Add Add-ons documentation to README.md

### Description of how you validated changes

None

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: badmintoncryer

packages/aws-cdk-lib/aws-eks/README.md

@@ -55,6 +55,7 @@ In addition, the library also supports defining Kubernetes resource manifests wi
       - [Manually importing k8s specs and CRD's](#manually-importing-k8s-specs-and-crds)
   - [Patching Kubernetes Resources](#patching-kubernetes-resources)
   - [Querying Kubernetes Resources](#querying-kubernetes-resources)
+  - [Add-ons](#add-ons)
   - [Using existing clusters](#using-existing-clusters)
   - [Logging](#logging)
   - [Known Issues and Limitations](#known-issues-and-limitations)
@@ -1837,6 +1838,20 @@ declare const cluster: eks.Cluster;
 const loadBalancerAddress = cluster.getServiceLoadBalancerAddress('my-service');
 ```
 
+## Add-ons
+
+[Add-ons](https://docs.aws.amazon.com/eks/latest/userguide/eks-add-ons.html) is a software that provides supporting operational capabilities to Kubernetes applications. The EKS module supports adding add-ons to your cluster using the `eks.Addon` class.
+
+```ts
+declare const cluster: eks.Cluster;
+
+new eks.Addon(this, 'Addon', {
+  cluster,
+  addonName: 'aws-guardduty-agent',
+  addonVersion: 'v1.6.1',
+});
+```
+
 ## Using existing clusters
 
 The Amazon EKS library allows defining Kubernetes resources such as [Kubernetes