Add sample krew plugin manifest

[ahmetb]

@soltysh has brought up this idea: why don't we have also include a Krew (https://sigs.k8s.io/krew) plugin manifest in this plugin sample?

To do this, we would need to:

1. Provide a prebuilt archive file containing cross-platform binaries of this repo (since this program is in Go) on GitHub releases section (or somewhere else on the internet).

2. Provide a command to built this archive file locally (make or hack/build-plugin-package.sh)

3. Add a sample-plugin.yaml to the root, that's pinning to a particular tag of the binary release archive and its sha256 sum (since we can't update those values as part of the release easily).

Once we do these, anyone cloning this repository can try the sample manifest like:

• Run make or hack/build-plugin-package.sh
• kubectl krew install --archive out/archive.tar.gz --manifest sample-plugin.yaml

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[corneliusweig]

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[ahmetb]

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