docs(README.md): Add jsr package registry prefix to add command

[christoshrousis]

PR Checklist

• Please ensure title follows conventional commits.
• Please add a description of your changes.
• We need your changes to come with unit tests in order to keep this project in quality and easy to maintain.
• If your changes contain any new feature please be sure to document it.
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Does this PR introduce a breaking change?

• Yes
• No

If yes please indicate it in the title and add migration info into the documentation.

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Need to add the registry prefix now: