doc(readme): Update readme with assistantv2 note

[ehdsouza]

No description provided.

[codecov-io]

Codecov Report

Merging #584 into master will not change coverage.
The diff coverage is n/a.

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[mediumTaj]

Erika it looks a little out of place under Examples. can you either put this in Before you begin like Mamoon did in the Ruby SDK or have the header like you have in Go?
https://github.com/watson-developer-cloud/go-sdk/blob/ea804c3813d8cd43e2324007d7c678d3abdd46f6/README.md#assistantv2

[germanattanasio]

@ehdsouza unless is necessary don't update responses.

[watson-github-bot]

🎉 This PR is included in version 2.3.0 🎉

The release is available on GitHub release

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