It's a Gradle project using the Gradle Wrapper, so you can use the standard gradle commands:
Build the plugin: ./gradlew build
Run unit tests: ./gradlew test
Package the sources-jar: ./gradlew kotlinSourcesJar
Package the kdoc-jar: ./gradlew javadocJar
Publish to your local maven repository: ./gradlew publishToMavenLocal.
This way you can test the plugin manually in another project by pulling your current SNAPSHOT from your local repository.