Update documention for MCP server integration

[bh-wise]

Update README.md following #2113. Closes #2120.

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.
• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've documented the impact of any changes to security controls.
  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.

[hashicorp-cla-app]

All committers have signed the CLA.

[github-actions]

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