atbash-cipher: add hint to @Ignore annotations

exercises/atbash-cipher/src/test/java/AtbashTest.java

@@ -63,7 +63,7 @@ public class AtbashTest {
             this.plaintext = plaintext;
         }
 
-        @Ignore
+        @Ignore("Remove to run test")
         @Test
         public void test() {
             assertEquals(plaintext, new Atbash().decode(ciphertext));