note that CRD models can be written manually

Author: yue9944882

docs/generate-model-from-third-party-resources.md

@@ -4,6 +4,8 @@ __TL;DR__: This document will be useful when you extend third-party resources in
 [CustomResourceDefinition](https://kubernetes.io/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions/)
 and try to program java to operate the extended APIs. The generation process requires your CRD to be defined with 
 [structral-schema](https://kubernetes.io/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions/#specifying-a-structural-schema).
+Alternatively, you can manually write your models for CRDs by implementing [KubernetesObject](https://github.com/kubernetes-client/java/blob/master/kubernetes/src/main/java/io/kubernetes/client/common/KubernetesObject.java) 
+and [KubernetesListObject](https://github.com/kubernetes-client/java/blob/master/kubernetes/src/main/java/io/kubernetes/client/common/KubernetesListObject.java) interfaces.
 
 
 ### Steps