docs: add recipe for using willGenerateGraphQLType with java enums (#712)

Author: lennyburdette

docs/writing-schemas/enums.md

@@ -22,3 +22,58 @@ enum MyEnumType {
   TWO
 }
 ```
+
+## Converting a Java enum to a GraphQL Enum
+
+If you want to use Java enums from another package, but you **don't** want
+include everything from that package using [`supportedPackages`][sp] or you want
+to customize the GraphQL type, you can use [schema generator hooks][hooks] to 
+associate the Java enum with a runtime [`GraphQLEnumType`][javadoc].
+
+[sp]:customizing-schemas/generator-config
+[hooks]:customizing-schemas/generator-config#schema-generator-hooks
+[javadoc]:https://javadoc.io/doc/com.graphql-java/graphql-java/latest/index.html
+
+Step 1: Create a GraphQLEnumType using the Java enum values
+
+```java
+// in some other package
+public enum Status {
+  APPROVED,
+  DECLINED
+}
+```
+
+```kotlin
+val statusEnumType = GraphQLEnumType.newEnum()
+    .name("Status")
+    .values(Status.values().map {
+      GraphQLEnumValueDefinition.newEnumValueDefinition()
+          .value(it.name)
+          .build()
+    })
+    .build()
+ ```
+ 
+ Step 2: Add a schema generation hook
+
+```kotlin
+class CustomSchemaGeneratorHooks : SchemaGeneratorHooks {
+
+  override fun willGenerateGraphQLType(type: KType): GraphQLType? {
+    return when (type.classifier as? KClass<*>) {
+      Status::class.java -> statusEnumType
+      else -> super.willGenerateGraphQLType(type)
+    }
+  }
+}
+```
+
+Step 3. Use your Java enum anywhere in your schema
+
+```kotlin
+@Component
+class StatusQuery : Query {
+  fun currentStatus: Status = getCurrentStatus()
+}
+```