Updated RuntimeEnvironments (markdown)

RuntimeEnvironments.md

@@ -9,3 +9,7 @@ Due to a [Python bug](http://bugs.python.org/issue10496), the interpreter
 requires the `$HOME` variable to be set to something. Otherwise, it tries to 
 look up the UID used by Isolate in `/etc/passwd` and fails. Setting `HOME=/box` 
 seems to work.
+
+## Java
+
+By default, we use a Groovy script that looks for a class with a `main` method and executes it for running Java programs. For this setup to work correctly, we need to set the `JAVA_HOME` environment variable and make sure that `$JAVA_HOME/jre/lib/amd64/jvm.cfg` is accessible from the isolate sandbox (the file can be a link to a file in a folder in `/etc`, which also needs to be bound to the sandbox).