Add example for groupMultiselect prompt to README file.

[Dmytro-Shulha]

Hello bombshell-dev team!
I was excited to find this tool and use it in my project!

I don't recall how exactly I've discovered groupMultiselect thing, but I think it should be mentioned on clack/prompts main page, just as other prompts.

This is why I created this pull request.

[changeset-bot]

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Latest commit: 285f5f5

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