Update README.md

Add test badge

Author: StarpTech

README.md

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+[![Lint and Test](https://github.com/wundergraph/cosmo-plugin-demo/actions/workflows/lint-test.yaml/badge.svg)](https://github.com/wundergraph/cosmo-plugin-demo/actions/workflows/lint-test.yaml)
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 ## ✨ Features
 
 - **Schema-First GraphQL**: Define your data contract in SDL and generate type-safe gRPC resolvers instantly