modified readme.txt (#1609)

3rdparty/readme.txt

-This folder contains libraries and headers of a few
-very popular still image codecs used by highgui.
-The libraries and headers are only to build Win32 and Win64 versions of OpenCV.
+This folder contains libraries and headers of a few very popular still image codecs
+used by highgui module.
+The libraries and headers are preferably to build Win32 and Win64 versions of OpenCV.
 On UNIX systems all the libraries are automatically detected by configure script.
+In order to use these versions of libraries instead of system ones on UNIX systems you
+should use BUILD_<library_name> CMake flags (for example, BUILD_PNG for the libpng library).
 
 ------------------------------------------------------------------------------------
 libjpeg 6b (6.2)  -   The Independent JPEG Group's JPEG software.
@@ -12,15 +14,15 @@ libjpeg 6b (6.2) - The Independent JPEG Group's JPEG software.
                       HAVE_JPEG preprocessor flag must be set to make highgui use libjpeg.
                       On UNIX systems configure script takes care of it.
 ------------------------------------------------------------------------------------
-libpng 1.4.3 - Portable Network Graphics library.
-               Copyright (C) 1998-2010, Glenn Randers-Pehrson.
+libpng 1.5.9      -   Portable Network Graphics library.
+                      Copyright (C) 2004, 2006-2011 Glenn Randers-Pehrson.
                       See libpng home page http://www.libpng.org
                       for details and links to the source code
 
                       HAVE_PNG preprocessor flag must be set to make highgui use libpng.
                       On UNIX systems configure script takes care of it.
 ------------------------------------------------------------------------------------
-libtiff 3.9.5 - Tag Image File Format (TIFF) Software
+libtiff 4.0.1     -   Tag Image File Format (TIFF) Software
                       Copyright (c) 1988-1997 Sam Leffler
                       Copyright (c) 1991-1997 Silicon Graphics, Inc.
                       See libtiff home page http://www.libtiff.org
@@ -28,13 +30,11 @@ libtiff 3.9.5 - Tag Image File Format (TIFF) Software
 
                       HAVE_TIFF preprocessor flag must be set to make highgui use libtiff.
                       On UNIX systems configure script takes care of it.
-
-                In this build support for ZIP (LZ77 compression), JPEG and LZW
-                are included.
+                      In this build support for ZIP (LZ77 compression) is turned on.
 ------------------------------------------------------------------------------------
-zlib 1.2.5 - General purpose LZ77 compression library
-             Copyright (C) 1995-2010 Jean-loup Gailly and Mark Adler.
-             See zlib home page http://www.gzip.org/zlib
+zlib 1.2.6        -   General purpose LZ77 compression library
+                      Copyright (C) 1995-2012 Jean-loup Gailly and Mark Adler.
+                      See zlib home page http://www.zlib.net
                       for details and links to the source code
 
                       No preprocessor definition is needed to make highgui use this library -