message: Update 3rdparty readme.txt for spng
diff: @@ -27,6 +27,13 @@ libpng Portable Network Graphics library.
  for details and links to the source code
  WITH_PNG CMake option must be ON to add libpng support to imgcodecs.
  +
  +libspng Portable Network Graphics library.
  + The license and copyright notes can be found in libspng/LICENSE.
  + See libspng home page https://www.libspng.org
  + for details and links to the source code
  +
  + WITH_SPNG CMake option must be ON to add libspng support to imgcodecs
  ------------------------------------------------------------------------------------
  libtiff Tag Image File Format (TIFF) Software
  Copyright (c) 1988-1997 Sam Leffler
  