add javadoc

src/main/java/core/RNG.java

@@ -3,6 +3,15 @@ package core;
 import java.security.SecureRandom;
 import java.util.Random;
 
+/**
+ * To ensure deterministic behaviour of repeating program executions,
+ * this class is used for all random number generation methods.
+ * Do not use Math.random()!
+ * It is not necessary to set a seed explicit, because a default one
+ * "123" is defined. Nonetheless a set-method is exposed which should
+ * ONLY be called in the very beginning of the program. (Do not reseed while
+ * execution)
+ */
 public class RNG {
     private static SecureRandom rng;
     private static int seed = 123;