Added documentation of how client-go interfaces with custom controller code

**What this PR does / why we need it**:

The sample-controller makes extensive use of various mechanisms
available in the client-go library. For writing custom controllers/operators
it will be helpful if there is precise description of how the
client-go library works and how/where it interfaces with
custom controller code.

This patch adds documentation that sheds light on how (parts) of the
client-go library work and its interaction-points with
controller code. The documentation and the diagram comes from [1],
(link included here for reference purpose).

[1] https://medium.com/@cloudark/kubernetes-custom-controllers-b6c7d0668fdf

Contributing the diagram and the writeup was recommended by
by @sttts and @nikhita on https://github.com/kubernetes/sample-controller/issues/13

**Release note**:

```release-note
NONE
```

staging/src/k8s.io/sample-controller/README.md

@@ -23,6 +23,13 @@ Changes should not be made to these files manually, and when creating your own
 controller based off of this implementation you should not copy these files and
 instead run the `update-codegen` script to generate your own.
 
+## Details
+
+The sample controller uses [client-go library](https://github.com/kubernetes/client-go/tree/master/tools/cache) extensively.
+The details of interaction points of the sample controller with various mechanisms from this library are
+explained [here](docs/controller-client-go.md).
+
+
 ## Purpose
 
 This is an example of how to build a kube-like controller with a single type.

staging/src/k8s.io/sample-controller/docs/controller-client-go.md

@@ -0,0 +1,64 @@
+# client-go under the hood
+
+The [client-go](https://github.com/kubernetes/client-go/) library contains various mechanisms that you can use when 
+developing your custom controllers. These mechanisms are defined in the 
+[tools/cache folder](https://github.com/kubernetes/client-go/tree/master/tools/cache) of the library.
+
+Here is a pictorial representation showing how the various components in 
+the client-go library work and their interaction points with the custom 
+controller code that you will write.
+
+<p align="center">
+  <img src="images/client-go-controller-interaction.jpeg" height="600" width="700"/>
+</p>
+
+## client-go components
+
+* Reflector: A reflector, which is defined in [type *Reflector* inside package *cache*](https://github.com/kubernetes/client-go/blob/master/tools/cache/reflector.go),
+watches the Kubernetes API for the specified resource type (kind).
+The function in which this is done is *ListAndWatch*.
+The watch could be for an in-built resource or it could be for a custom resource.
+When the reflector receives notification about existence of new
+resource instance through the watch API, it gets the newly created object 
+using the corresponding listing API and puts it in the Delta Fifo queue
+inside the *watchHandler* function.
+
+
+* Informer: An informer defined in the [base controller inside package *cache*](https://github.com/kubernetes/client-go/blob/master/tools/cache/controller.go) pops objects from the Delta Fifo queue.
+The function in which this is done is *processLoop*. The job of this base controller
+is to save the object for later retrieval, and to invoke our controller passing it the object.
+
+* Indexer: An indexer provides indexing functionality over objects.
+It is defined in [type *Indexer* inside package *cache*](https://github.com/kubernetes/client-go/blob/master/tools/cache/index.go). A typical indexing use-case is to create an index based on object labels. Indexer can
+maintain indexes based on several indexing functions.
+Indexer uses a thread-safe data store to store objects and their keys.
+There is a default function named *MetaNamespaceKeyFunc* defined in [type Store inside package cache](https://github.com/kubernetes/client-go/blob/master/tools/cache/store.go)
+that generates an object’s key as `<namespace>/<name>` combination for that object.
+
+
+## Custom Controller components
+
+* Informer reference: This is the reference to the Informer instance that knows 
+how to work with your custom resource objects. Your custom controller code needs 
+to create the appropriate Informer.
+
+* Indexer reference: This is the reference to the Indexer instance that knows 
+how to work with your custom resource objects. Your custom controller code needs 
+to create this. You will be using this reference for retrieving objects for 
+later processing.
+
+The base controller in client-go provides the *NewIndexerInformer* function to create Informer and Indexer.
+In your code you can either [directly invoke this function](https://github.com/kubernetes/client-go/blob/master/examples/workqueue/main.go#L174) or [use factory methods for creating an informer.](https://github.com/kubernetes/sample-controller/blob/master/main.go#L61)
+
+* Resource Event Handlers: These are the callback functions which will be called by 
+the Informer when it wants to deliver an object to your controller. The typical 
+pattern to write these functions is to obtain the dispatched object’s key
+and enqueue that key in a work queue for further processing.
+
+* Work queue: This is the queue that you create in your controller code to decouple 
+delivery of an object from its processing. Resource event handler functions are written 
+to extract the delivered object’s key and add that to the work queue.
+
+* Process Item: This is the function that you create in your code which processes items 
+from the work queue. There can be one or more other functions that do the actual processing. 
+These functions will typically use the [Indexer reference](https://github.com/kubernetes/client-go/blob/master/examples/workqueue/main.go#L73), or a Listing wrapper to retrieve the object corresponding to the key.