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]

⚠️ No Changeset found

Latest commit: 24d3ab6

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[pkg-pr-new]

@example/basic • @example/changesets

npm i https://pkg.pr.new/bombshell-dev/clack/@clack/core@215

npm i https://pkg.pr.new/bombshell-dev/clack/@clack/prompts@215

commit: 24d3ab6