chore: update README with shell, stockfish and RAE servers

[sonirico]

Adds 3 MCP servers

Motivation and Context

• shell: lets your LLM run commands in your shell freely. Support for docker to provide isolation out of the box.
• RAE: improves your LLM Spanish knowledger by giving it access to official Spanish definitions, conjugations and more.
• Stockfish: So that LLMs can finally play chess without spawning non-existent pieces or sacrificing your king.

How Has This Been Tested?

Used Claude Desktop

Breaking Changes

No

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)
• [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