README: add Homebrew MCP server.

[MikeMcQuaid]

Description

Motivation and Context

This is now part of Homebrew so let's include it here.

How Has This Been Tested?

Yes, Cursor and Claude.

Breaking Changes

Required configuration:

{
  "mcpServers": {
    "homebrew": {
      "command": "brew",
      "args": [
        "mcp-server"
      ]
    }
  }
}

Types of changes

• [x] Documentation update

Checklist

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

Additional context

Feedback welcome on this PR or the Homebrew MCP server in general!