Add JSDelivr CDN badge to README

Author: huhuhang

README.md

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 # LabEx Schema
 
+[![](https://data.jsdelivr.com/v1/package/gh/labex-labs/schema/badge)](https://www.jsdelivr.com/package/gh/labex-labs/schema)
+
 This repository contains the JSON Schema definitions for LabEx configuration files. These schemas define the structure and validation rules for lab and course configurations in the LabEx platform.
 
 ## index.json