phone-number: add hint to @Ignore annotations

exercises/phone-number/src/test/java/PhoneNumberTest.java

         );
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void cleansNumberWithDots() {
         final String expectedNumber = "1234567890";
         );
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void cleansNumberWithMultipleSpaces() {
         final String expectedNumber = "1234567890";
         );
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void invalidWhen9Digits() {
         expectedException.expect(IllegalArgumentException.class);
         new PhoneNumber("123456789");
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void invalidWhen11Digits() {
         expectedException.expect(IllegalArgumentException.class);
         new PhoneNumber("21234567890");
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void validWhen11DigitsAndFirstIs1() {
         final String expectedNumber = "1234567890";
         );
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void invalidWhen12Digits() {
         expectedException.expect(IllegalArgumentException.class);
         new PhoneNumber("321234567890");
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void invalidWithLetters() {
         expectedException.expect(IllegalArgumentException.class);
         new PhoneNumber("123-abc-7890");
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void invalidWithPunctuation() {
         expectedException.expect(IllegalArgumentException.class);
         new PhoneNumber("123-@:!-7890");
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void invalidWithRightNumberOfDigitsButLettersMixedIn() {
         expectedException.expect(IllegalArgumentException.class);