Add a comment about a patch to be able to reda wrong images, wher UN length is

Add a comment about a patch to be able to reda wrong images, wher UN length is
coded on 2 bytes.
   // WARNING :
   //
   // For some images, length of UN elements is coded on 2 bytes (instead of 4)
   // There are *not* readable !
   // You can make a quick and dirty patch, commenting out
   //| vr == "UN"
   // in the following line.
   // (the 'straight' images will no longer be readable ...)

      if ( vr == "OB" || vr == "OW" || vr == "SQ" || vr == "UT"
                                                          || vr == "UN" || changeFromUN == true)
      {