docs: add Factory CLI configuration to MCP clients

[iannuttall]

This PR adds Factory CLI to the MCP client configuration section in the README.

The Factory CLI section:

• Follows the same format as other MCP clients
• Is placed in alphabetical order (between Cursor and Gemini CLI)
• Includes a link to the Factory documentation
• Provides the command to add the Chrome DevTools MCP server using droid mcp add

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