[DOCS-3652] Note LINQ API is in preview

[jrodewig]

Description

Notes that the LINQ functionality is in preview and may change in future releases.

Motivation and context

Sets users expectations about support while in beta.

We're removing LINQ refs from docs.fauna.com with https://github.com/fauna/docs.fauna.com/pull/2533.

How was the change tested?

N/A

Screenshots (if appropriate):

Change types

• • [ ] Bug fix (non-breaking change that fixes an issue)
• • [ ] New feature (non-breaking change that adds functionality)
• • [ ] Breaking change (backwards-incompatible fix or feature)

Checklist:

• • [ ] My code follows the code style of this project.
• • [x] My change requires a change to Fauna documentation.
• • [x] My change requires a change to the README, and I have updated it accordingly.

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