**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
```
Kubernetes-commit: 3e22383a8a14eb3bd02630f3f11c0ad7a806e027
