[doc] Add preview note to README and registry

[aureq]

What happened?

Let's ensure that the README clearly indicates that the Pulumi Service provider is in public preview. Also let's make sure this shows up in the Pulumi registry like it's currently done for the AWS native provider.

Example

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Output of pulumi about

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Additional context

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Contributing

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[IaroslavTitov]

Working on PSPV1 to not need the preview info, but can implement for now.

Looking into it noticed we do need to update registry landing page for PSP though, the name is confusing - https://www.pulumi.com/registry/packages/pulumiservice/