Improve Gradle Kotlin DSL compatibility

Hi!

I'm switching over in my Gradle build files from Groovy (build.groovy) to the Gradle Kotlin DSL (build.groovy.kts).

(using Gradle 5.4.1)

**It is not obvious** IMHO, that the Groovy version of the appengine configuration section (build.gradle)

```
appengine {
tools {
// configure the Cloud Sdk tooling
}
stage {
// configure staging for deployment
}
deploy {
// configure deployment
}
}
```

must be written (AFAIK) like this, using the Gradle Kotlin DSL (build.gradle.kts)

```
import com.google.cloud.tools.gradle.appengine.appyaml.AppEngineAppYamlExtension

// ...

the().apply {
tools {
// configure the Cloud Sdk tooling
}
stage {
// configure staging for deployment
}
deploy {
// configure deployment
}
}
```

Could you add some Gradle Kotlin DSL examples somewhere in the https://github.com/GoogleCloudPlatform/app-gradle-plugin/blob/master/USER_GUIDE.md ?

Thanks a lot and best regards,
Peti