Add status badge to readme

Author: kgbier

README.md

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 # graphql-parser
 ### Konrad Biernacki (<kgbier@gmail.com>)
 
+[![Gradle](https://github.com/kgbier/graphql-parser-kotlin/actions/workflows/gradle.yml/badge.svg)](https://github.com/kgbier/graphql-parser-kotlin/actions/workflows/gradle.yml)
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 A utility for parsing GraphQL queries. Written with help from the excellent 
 [Point·Free](https://www.pointfree.co/collections/parsing).