Adding doc for client-gen

cmd/libs/go2idl/client-gen/README.md

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+See [generating-clientset.md](../../../../docs/devel/generating-clientset.md)
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+[![Analytics](https://kubernetes-site.appspot.com/UA-36037335-10/GitHub/cmd/libs/go2idl/client-gen/README.md?pixel)]()

docs/devel/generating-clientset.md

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+<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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+<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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+<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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+<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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+<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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+<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>
+
+If you are using a released version of Kubernetes, you should
+refer to the docs that go with that version.
+
+Documentation for other releases can be found at
+[releases.k8s.io](http://releases.k8s.io).
+</strong>
+--
+
+<!-- END STRIP_FOR_RELEASE -->
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+<!-- END MUNGE: UNVERSIONED_WARNING -->
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+# Generation and release cycle of clientset
+
+Client-gen is an automatic tool that generates [clientset](../../docs/proposals/client-package-structure.md#high-level-client-sets) based on API types. This doc introduces the use the client-gen, and the release cycle of the generated clientsets.
+
+## Using client-gen
+
+The workflow includes four steps:
+- Marking API types with tags: in `pkg/apis/${GROUP}/${VERSION}/types.go`, mark the types (e.g., Pods) that you want to generate clients for with the `// +genclient=true` tag.  If the resource associated with the type is not namespace scoped (e.g., PersistentVolume), you need to append the `nonNamespaced=true` tag as well.
+- Running the client-gen tool: you need to use the command line argument `--input` to specify the groups and versions of the APIs you want to generate clients for, client-gen will then look into `pkg/apis/${GROUP}/${VERSION}/types.go` and generate clients for the types you have marked with the `genlcient` tags. For example, run
+
+```
+$ client-gen --input="api/v1,extensions/v1beta1" --clientset-name="my_release"
+```
+
+will generate a clientset named "my_release" which includes clients for api/v1 objects and extensions/v1beta1 objects. You can run `$ client-gen --help` to see other command line arguments.
+- Adding expansion methods: client-gen only generates the common methods, such as `Create()` and `Delete()`. You can manually add additional methods through the expansion interface. For example, this [file](../../pkg/client/typed/generated/core/v1/pod_expansion.go) adds additional methods to Pod's client. As a convention, we put the expansion interface and its methods in file ${TYPE}_expansion.go.
+- Generating Fake clients for testing purposes: client-gen will generate a fake clientset if the command line argument `--fake-clientset` is set. The fake clientset provides the default implementation, you only need to fake out the methods you care about when writing test cases.
+
+The output of client-gen inlcudes:
+- Individual typed clients and client for group: They will be generated at `pkg/client/typed/generated/${GROUP}/${VERSION}/`
+- clientset: the top-level clientset will be generated at `pkg/client/clientset_generated` by default, and you can change the path via the `--clientset-path` command line argument.
+
+## Released clientsets
+
+At the 1.2 release, we have two released clientsets in the repo: internalclientset and release_1_2.
+- internalclientset: because most components in our repo still deal with the internal objects, the internalclientset talks in internal objects to ease the adoption of clientset. We will keep updating it as our API evolves. Eventually it will be replaced by a versioned clientset.
+- release_1_2: release_1_2 clientset is a versioned clientset, it includes clients for the core v1 objects, extensions/v1beta1, autoscaling/v1, and batch/v1 objects. We will NOT update it after we cut the 1.2 release. After the 1.2 release, we will create release_1_3 clientset and keep it updated until we cut release 1.3.
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+<!-- BEGIN MUNGE: GENERATED_ANALYTICS -->
+[![Analytics](https://kubernetes-site.appspot.com/UA-36037335-10/GitHub/docs/devel/generating-clientset.md?pixel)]()
+<!-- END MUNGE: GENERATED_ANALYTICS -->