Update README.md with pnpm installation guide

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Name	Status	Preview	Comments	Updated
prettier-plugin-go-template	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Feb 28, 2023 at 10:03PM (UTC)

[mtorp]

@NiklasPor any chance you can merge this soon?

[NiklasPor]

Thanks for the contribution, actually this isn't needed anymore as with Prettier 3 every plugin needs to be explicitly mentioned. I added a small note at the beginning of the readme 👍