add example code of generating Zsh completion

[autopp]

Current zsh_completion.md only shows what cobra supports for generating Zsh completion, not how to do id. So I added basic usage.

I hope this help.

[CLAassistant]

All committers have signed the CLA.

[wfernandes]

@autopp Thanks for this PR.
@jpmcb Can you take a look at this since you recently did some zsh testing? 😉

[jpmcb]

Hi @autopp thanks for this PR!

I'm thinking this may have to wait a bit for this to get merged: #1070.

With #1035 being merged and #1048 also getting in, looks like zsh will be the next to get the "Go completions" facelift. So stay tuned! I'll check back in with this PR after those are merged

[github-actions]

This PR is being marked as stale due to a long period of inactivity

[marckhouzam]

@autopp The documentation for shell completion has extensively changed recently. An example of generating the different completion scripts can be found here:
https://github.com/spf13/cobra/blame/884edc58ad08083e6c9a505041695aa2c3ca2d7a/shell_completions.md#L56

If you find that the doc now addresses your concern, would you mind closing this PR? Thank you.

[autopp]

@marckhouzam The current doc seems to address my concern. Thanks!