Included warning and temporary fix for kotlin 1.3 dependency

Author: oliemansm

README.md

@@ -10,6 +10,29 @@ GraphQL Java Tools works extremely well if you already have domain POJOs that ho
 
 GraphQL Java Tools aims for seamless integration with Java, but works for any JVM language.  Try it with Kotlin!
 
+## WARNING: NoClassDefFoundError when using Spring Boot
+
+If you're using `graphl-java-tools` you need to set the `kotlin.version` in your Spring Boot project explicitly to 
+version 1.3.10, because Spring Boot Starter parent currently overrides it with a 1.2.* version of Kotlin. 
+`graphql-java-tools` requires 1.3.* however because of its coroutine support. If you don't override this version
+you will run into a `NoClassDefFoundError`.
+
+Spring Boot team has indicated the Kotlin version will be upgraded to 1.3 in Spring Boot 2.2.
+
+### Using Gradle
+Set the Kotlin version in your `gradle.properties`
+```
+kotlin.version=1.3.10
+```
+
+### Using Maven
+Set the Kotlin version in your `<properties>` section
+```xml
+<properties>
+  <kotlin.version>1.3.10</kotlin.version>
+</properties>
+```
+
 ## Documentation
 
 Take a look at our new [documentation](https://www.graphql-java-kickstart.com/tools/) for more details.