Add some comment to skipped tests

Author: mirfilip

tests/Drafts/Draft3Test.php

@@ -31,6 +31,7 @@ protected function getFilePaths()
     protected function getSkippedTests()
     {
         return array(
+            // Optional
             'bignum.json',
             'format.json',
             'jsregex.json',

tests/Drafts/Draft4Test.php

@@ -36,7 +36,7 @@ protected function getSkippedTests()
             'format.json',
             'zeroTerminatedFloats.json',
             // Required
-            'not.json'
+            'not.json' // only one test case failing
         );
     }
 }