Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Adds a build example with KUBE_BUILD_PLATFORMS

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

I didn't manage to find any guides about how to build the specific binary for the specific platform in docs (in this repo and in the community repo). Also it was quite hard for me to follow build scripts. At the end, I found help on the kubernetes-dev channel in Slack:`KUBE_BUILD_PLATFORMS` seems to do what I want. I think, would be great to have it documented somehow: The "Key scripts" section of the build doc seems to be a good place to mention it.

**Release note**:

```release-note
NONE
```