Add note to largest-series-product tests about JUnit Rules

exercises/largest-series-product/src/test/java/LargestSeriesProductCalculatorTest.java

 
 public final class LargestSeriesProductCalculatorTest {
 
+    /*
+     * 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();