Update README.md: multi-node configuration

[phiHero]

Change Summary

New client with multi-node configuration options:

client := typesense.NewClient(
        typesense.WithNearestNode("https://xxx.a1.typesense.net:443"),
        typesense.WithNodes([]string{
            "https://xxx-1.a1.typesense.net:443",
            "https://xxx-2.a1.typesense.net:443",
            "https://xxx-3.a1.typesense.net:443",
        }),
        typesense.WithAPIKey("<API_KEY>"),
        typesense.WithNumRetries(5),
        typesense.WithRetryInterval(1*time.Second),
        typesense.WithHealthcheckInterval(2*time.Minute),
    )

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