Add link to Kotlin sample for Spring Cloud GCP

Closes gh-2070

Author: dzou

src/docs/asciidoc/languages/kotlin.adoc

@@ -755,6 +755,7 @@ The following Github projects offer examples that you can learn from and possibl
 * https://github.com/sdeleuze/spring-kotlin-fullstack[spring-kotlin-fullstack]: WebFlux Kotlin fullstack example with Kotlin2js for frontend instead of JavaScript or TypeScript
 * https://github.com/spring-petclinic/spring-petclinic-kotlin[spring-petclinic-kotlin]: Kotlin version of the Spring PetClinic Sample Application
 * https://github.com/sdeleuze/spring-kotlin-deepdive[spring-kotlin-deepdive]: A step-by-step migration guide for Boot 1.0 and Java to Boot 2.0 and Kotlin
+* https://github.com/spring-cloud/spring-cloud-gcp/tree/master/spring-cloud-gcp-kotlin-samples/spring-cloud-gcp-kotlin-app-sample[spring-cloud-gcp-kotlin-app-sample]: Spring Boot with Google Cloud Platform Integrations