Add Customer.io integration to README

[scotttwittrockcio]

Description

This PR adds the Customer.io MCP server to the list of servers.

Server Details

• Server: Customer.io MCP Server
• Changes to: README.md (add new community server reference)

Motivation and Context

This change adds visibility for the Customer.io MCP Server, allowing the MCP community to discover and use a server that provides comprehensive customer data platform capabilities. The server enables users to work directly with Customer.io workspaces through Claude to create segments, inspect user profiles, search for customers, and access workspace data without switching tabs. It expands the ecosystem of available MCP servers and demonstrates integration with LLM clients for enhanced customer intelligence and audience targeting workflows.

How Has This Been Tested?

The Customer.io MCP Server has been extensively tested with Claude Desktop, Claude Code, and ChatGPT to ensure proper functionality for:

• Creating customer segments based on behavioral and attribute criteria
• Retrieving detailed user profiles with attributes and engagement history
• Searching and exploring workspace data including custom attributes and existing segments
• OAuth authentication flow and workspace access permissions

Breaking Changes

No breaking changes. This is an addition to the community servers list. Users who wish to use the Customer.io MCP Server will need to add its configuration to their MCP client and authenticate with their Customer.io workspace.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context

The Customer.io MCP Server supports both US and EU regions with dedicated endpoints, implements OAuth 6/18 auth spec for secure workspace access, and provides comprehensive documentation at https://docs.customer.io/ai/mcp-server/. The server includes tools for workspace management (create, get, list), customer data exploration (search), and SDK integration support, making it a valuable addition for marketing teams and customer success professionals using Claude for customer intelligence workflows.