Added Gradle badge

Author: gazbert

README.md

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 # Java Design Patterns
 
+[![Gradle CI](https://github.com/gazbert/java-design-patterns/actions/workflows/gradle.yml/badge.svg?branch=master)](https://github.com/gazbert/java-design-patterns/actions/workflows/gradle.yml) 
 [![Maven CI](https://github.com/gazbert/java-design-patterns/actions/workflows/maven.yml/badge.svg?branch=master)](https://github.com/gazbert/java-design-patterns/actions/workflows/maven.yml)
 
 A cookbook of commonly used Java design patterns based on examples from: