Add Bidi-Mirrored letter list link as comment

Bug: 5047217
Change-Id: I29449892803c5150726aebfe97ecef6d481e1e97

java/src/com/android/inputmethod/keyboard/Key.java

@@ -159,6 +159,8 @@ public class Key {
     private static final Map<Integer, Integer> sRtlParenthesisMap = new HashMap<Integer, Integer>();
 
     static {
+        // The all letters need to be mirrored are found at
+        // http://www.unicode.org/Public/6.0.0/ucd/extracted/DerivedBinaryProperties.txt
         addRtlParenthesisPair('(', ')');
         addRtlParenthesisPair('[', ']');
         addRtlParenthesisPair('{', '}');