Add image to README for visual enhancement

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Author: yotsuda

README.md

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 PowerShell.MCP connects AI assistants to the entire PowerShell ecosystem through a single MCP server. You and AI collaborate in the same PowerShell console—every command is visible, auditable, and saved to history.
 
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 ## Why PowerShell.MCP?
 
 **A universal gateway to the PowerShell ecosystem.**