Documentation is added in several areas:
1. `kubectl plugin` now prints a note that plugins are best discovered
with krew.dev and how to install it.
2. The kubectl book now has a new section about plugins, featuring
- a very brief introduction to the kubectl plugin mechanism
- a section about krew
Kubernetes-commit: ed0e0350854e34bad65de4db44c0f661a5851870
Cornelius Weig