Add note re OpenType 1.7 language tags

src/hb-ot-tag.cc

@@ -175,6 +175,11 @@ typedef struct {
  *
  * Some items still missing.  Those are commented out at the end.
  * Keep sorted for bsearch.
+ *
+ * Updated as of 2015-05-06: OT1.7 on MS website has some newer
+ * items that we don't have here, eg. Zazaki.  This is the new
+ * items in OpenType 1.7 (red items), most of which we have:
+ * http://www.microsoft.com/typography/otspec170/languagetags.htm
  */
 
 static const LangTag ot_languages[] = {