browser-sdk: add v3 changes to README

[kevinwhereby]

---

Description

Summary:
just a quick readme update showing the v3 api on the browser-sdk readme after some feedback in the support Discord

Related Issue:
https://linear.app/whereby/issue/PAN-1442/update-react-sdk-docs

Testing

Screenshots/GIFs (if applicable)

Checklist

• My code follows the project's coding standards.
• Prefixed the PR title and commit messages with the service or package name
• I have written unit tests (if applicable).
• I have updated the documentation (if applicable).
• By submitting this pull request, I confirm that my contribution is made
  under the terms of the MIT license.

Additional Information

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Latest commit: 5819b10

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[thyal]

Nice