Update ADS readme

[andyleejordan]

Prerequisites

• I have written a descriptive issue title.
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Summary

The document that ADS uses for its marketplace is woefully out-of-date, we need to update it: https://github.com/PowerShell/vscode-powershell/blob/master/docs/azure_data_studio/README_FOR_MARKETPLACE.md

Proposed Design

No response