Add completion to help message

[navidshaikh]

Fixes #477

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-client-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/kn/commands/completion/completion.go	Do not exist	100.0%

[rhuss]

Thanks! Looks good to me, but please let's check whether we should do it as we intentionally removed it with #263

How do folks feel to include kn completion again in the top-level kn help output?

My take is, that we should add it with this PR and not treat it as a magic gem (especially as we are advertising it on our main README, which is another topic on its own). Since we have now more top-level commands (and more to come with source and trigger), I think it's not so disturbing anymore.

[maximilien]

I assume this goes after Zsh completion is added again?

[maximilien]

*"which needs"

[rhuss]

Suggested change

	This command prints shell completion code which need to be evaluated
	This command prints shell completion code which needs to be evaluated

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: maximilien, navidshaikh

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[rhuss]

Let's get that merged.
@navidshaikh if you just fix this minor typo, then I will do a lgtm right after.

[rhuss]

/lgtm