Instruction on how to enable cross compilation

src/genjava/templates/genjava_project/build.gradle.in

@@ -67,6 +67,12 @@ jar {
     }
 }
 
+/* Uncomment the following lines in package://genjava/templates/genjava_project/build.gradle.in to enable Java 6
+ * cross compilation for ROS messages.
+ */
+// sourceCompatibility = 1.6
+// targetCompatibility = 1.6
+
 task info {
     doLast {
         println "\nProject Info"