docs: restructure development documentation and add custom tools guide

Rearranged the development section in README.md to better organize the documentation flow. Added a dedicated section for "Adding Custom Tools" with a link to the new CUSTOM_TOOLS.md file, and renamed the previous "For Developers" section to "Contributing to the Project" to better reflect its content. This improves discoverability and organization of the development setup documentation.

Author: msanatan

README-zh.md

@@ -270,7 +270,11 @@ claude mcp add UnityMCP -- "C:/Users/USERNAME/AppData/Local/Microsoft/WinGet/Lin
 
 ## 开发和贡献 🛠️
 
-### 开发者
+### 添加自定义工具
+
+MCP for Unity 使用与 Unity 的 C# 脚本绑定的 Python MCP 服务器来实现工具功能。如果您想使用自己的工具扩展功能，请参阅 **[CUSTOM_TOOLS.md](docs/CUSTOM_TOOLS.md)** 了解如何操作。
+
+### 贡献项目
 
 如果您正在为 MCP for Unity 做贡献或想要测试核心更改，我们有开发工具来简化您的工作流程：
 

README.md

@@ -273,7 +273,11 @@ On Windows, set `command` to the absolute shim, e.g. `C:\\Users\\YOU\\AppData\\L
 
 ## Development & Contributing 🛠️
 
-### For Developers
+### Adding Custom Tools
+
+MCP for Unity uses a Python MCP Server tied with Unity's C# scripts for tools. If you'd like to extend the functionality with your own tools, learn how to do so in **[CUSTOM_TOOLS.md](docs/CUSTOM_TOOLS.md)**.
+
+### Contributing to the Project
 
 If you're contributing to MCP for Unity or want to test core changes, we have development tools to streamline your workflow: