add graphql-kotlin link to the list of GraphQL clients

Add link to https://github.com/ExpediaGroup/graphql-kotlin/ to a list of GraphQL Kotlin clients.

Author: dariuszkuc

site/code/index.html.js

@@ -616,6 +616,7 @@ Executor.execute(schema, query) map println
 - [Java / Android](#java-android)
 - [JavaScript](#javascript-1)
 - [Julia](#julia)
+- [Kotlin](#kotlin)
 - [Swift / Objective-C iOS](#swift-objective-c-ios)
 - [Python](#python-1)
 - [R](#r)
@@ -665,6 +666,10 @@ Executor.execute(schema, query) map println
 
   - [Diana.jl](https://github.com/codeneomatrix/Diana.jl): A Julia GraphQL server implementation.
 
+### Kotlin
+
+  - [graphql-kotlin](https://github.com/ExpediaGroup/graphql-kotlin/): A set of GraphQL libraries that includes a lightweight, typesafe GraphQL HTTP client.
+
 ### Swift / Objective-C iOS
 
   - [Apollo iOS](https://www.apollographql.com/docs/ios/) ([github](https://github.com/apollographql/apollo-ios)): A GraphQL client for iOS that returns results as query-specific Swift types, and integrates with Xcode to show your Swift source and GraphQL side by side, with inline validation errors.