Fixed operator< on ZString.

ZString.h

@@ -100,6 +100,10 @@ namespace coreutils {
       /// A friend method used to write the value of the ZString
       /// to an ostream using the << syntax.
       ///
+
+      ///
+      /// The < operator is used to compare values if ZString is used in std::map.
+      ///
       
       bool operator<(const ZString &valuex) const;