Spring Add a link to Kotlin Sample w/ Spring Cloud GCP

This add a Kotlin Spring Boot sample demonstrating integrations with Google Cloud Platform.

Hi, just to clarify - I work with the Spring Cloud GCP team at Google; we work with Pivotal to build integrations for Spring and Google Cloud. This PR is just to add a plug to a Kotlin sample application that integrates with Google Cloud. Not sure if this example fits within your docs but figured it s worth a shot.

cc/ @meltsufin