Update README.md

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This update enhances the SharePoint site inventory script by expanding the data collected for each site. The script now includes site creation date, optional last activity information, and optional external sharing status, providing greater visibility into site age, usage, and potential security exposure. Performance has been improved by reducing redundant tenant queries and consolidating data retrieval, making the script more suitable for larger Microsoft 365 environments. These changes improve the script’s usefulness for governance, security reviews, and lifecycle management.