Add badge

Author: jcs090218

README.md

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 # shader-language-server
 > Language server implementation for ShaderLab
 
+[![Build](https://github.com/shader-ls/shader-language-server/actions/workflows/test.yml/badge.svg)](https://github.com/shader-ls/shader-language-server/actions/workflows/test.yml)
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 ## 🚧 Status
 
 Still in development.