added note about Java 9 not being fully supported

gradle/content.md

@@ -11,3 +11,5 @@
 Run this from the directory of the Gradle project you want to build.
 
 `docker run --rm -v "$PWD":/project -w /project --name gradle gradle:alpine gradle <gradle-task>`
+
+**Note: Java 9 support is experimental**

groovy/content.md

@@ -22,3 +22,5 @@ The local Grapes cache can be reused across containers by creating a volume and
 docker volume create --name grapes-cache
 docker run -it -v grapes-cache:/home/groovy/.groovy/grapes groovy:alpine
 ```
+
+**Note: Java 9 support is experimental**