Add maintenance disclaimer to README.

Author: jeggy

README.MD

@@ -3,6 +3,9 @@
 [![Awesome Kotlin Badge](https://kotlin.link/awesome-kotlin.svg)](https://github.com/KotlinBy/awesome-kotlin)
 [![Chat](https://img.shields.io/badge/chat-on%20slack-green.svg)](https://kotlinlang.slack.com/messages/kgraphql)
 
+# Disclaimer
+We are no longer able to maintain this project. Please go to https://github.com/stuebingerb/KGraphQL, where KGraphQL is still developed.
+
 KGraphQL is a [Kotlin](https://kotlinlang.org/) implementation of [GraphQL](http://graphql.org/). It provides a rich DSL to set up the GraphQL schema.