Added Unity-MCP to the Community Servers section

[IvanMurzak]

Description

Added Unity-MCP to the community servers section of the README.md

Motivation and Context

Unity-MCP as MCP integration with Unity Engine (game engine). It helps to develop games in many ways. Run tests, read and modify Unity Assets, operate in Unity environment in many ways.

How Has This Been Tested?

I am using it for months during my personal game development tasks in Unity Engine.
You may view gif demos here, and videos here.

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