chore(contributing.adoc): Add explanation to add KOTLIN_JDK_HOME for …

…building on MacOs

As this seems to be a temporary workaround, just let document it.
Otherwise it would have made sense to adapt the Makefile to set it
automatically.

Fixes apache#297.

contributing.adoc

@@ -72,6 +72,16 @@ make dep
 
 The `make dep` command runs `dep ensure -v` under the hood, so make sure that `dep` is properly installed.
 
+For now, you have to set environment variable `KOTLIN_JDK_HOME` pointing to your JDK (not JRE) home directory.
+Also ensure, that you are using a Java version 8.
+
+For Mac you can just do a
+
+[source, bash]
+----
+export KOTLIN_JDK_HOME=$(/usr/libexec/java_home -v 1.8)
+----
+
 To build the whole project you now need to run:
 
 ```
@@ -182,4 +192,4 @@ After you setup the IDE task, you can run and debug the operator process.
 
 NOTE: The operator can be fully debugged in Minishift, because it uses OpenShift S2I binary builds under the hood.
 The build phase cannot be (currently) debugged in Minikube because the Kaniko builder requires that the operator and the publisher pod
-share a common persistent volume.
+share a common persistent volume.