Revise README for better structure and content

Updated the README to improve clarity and formatting.

Author: LgE02

README.md

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+# 📱 Routine Master
+Routine Master의 백엔드 레포지토리입니다.
+
+<br>
+
+## 🛠️ 기술 스택 (Tech Stack)
+
+| 기술        | 설명|
+|------------| --- |
+| Java       | v21 |
+| Framework   | Spring Boot 3.5.7|
+| Database | MySQLD|
+| ORM | JPA |
+
+
+<br>
+
+## 🌿 프로젝트 구조
+<img width="500" height="auto" alt="백" src="https://github.com/user-attachments/assets/f9006c6b-a23a-4d58-a455-f805fb707a86" />
+
+