Add context r.e. JUnit Rules to queen-attack tests

exercises/queen-attack/src/test/java/QueenAttackCalculatorTest.java

@@ -8,6 +8,10 @@ import static org.junit.Assert.assertTrue;
 
 public final class QueenAttackCalculatorTest {
 
+    /*
+     * See https://github.com/junit-team/junit4/wiki/Rules for information on JUnit Rules in general and
+     * ExpectedExceptions in particular.
+     */
     @Rule
     public ExpectedException expectedException = ExpectedException.none();