docs: 리드미 추가 - 초기 성능

Author: solbijae

README.md

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+# 바닐라 JS 프로젝트 성능 개선
+- url: https://front-5th-chapter4-2-basic.netlify.app/
+
+## 성능 개선 보고서
+### 초기 성능
+- PageSpeed Insights 보고서 링크: 
+  모바일: https://pagespeed.web.dev/analysis/https-front-5th-chapter4-2-basic-netlify-app/mjziitvm5k?hl=ko&form_factor=mobile
+  데스크톱: https://pagespeed.web.dev/analysis/https-front-5th-chapter4-2-basic-netlify-app/mjziitvm5k?hl=ko&form_factor=desktop
+- 보고서 전체 스크린샷
+    <details>
+        <summary>모바일</summary>
+        ![Image](https://github.com/user-attachments/assets/2d9b2d72-c845-47f6-9488-8f6f5ba8e191)
+    </details>
+    <details>
+        <summary>데스크톱</summary>
+        ![Image](https://github.com/user-attachments/assets/4c27af8c-a720-42ad-880d-b654524749d3)
+    </details>