Add CImg usage in to the read me file.

Author: wutipong

README.md

@@ -1,11 +1,13 @@
 # ZipPicViewWx
 
 ![Imgur](http://i.imgur.com/4DRWcjM.png)
+
 ![Imgur](http://i.imgur.com/tQ2UIEC.png)
 
 ZipPicViewWx is an in-zip-archive image viewer. It is written in C++ and runs on Linux and Windows (OSX isn't tested yet). It has only a basic feature like image listing, view with zoom-in/out (up-to 200%), and does not aim to replace your day-to-day image viewer.
 
 ![Imgur](http://i.imgur.com/p3i5G5w.png)
+
 ![Imgur](http://i.imgur.com/Xdjv6R0.png)
 
 ZipPicViewWx has been tested on Windows 10 and Arch Linux.
@@ -19,10 +21,16 @@ The tested method is to use UNIX toolchain. ZipPicViewWx use CMake for build sys
 3. Change directory into the build directory. And then run `$ cmake ~/ZipPicViewWx`.
 4. Then run `$ make`. The program is now built.
 
-On Windows, [MSYS2](https://msys2.github.io/) is the preferred method. Install MSYS2 and necessary toolchain, then follow the same instruction as the UNIX toolchain. The Generator has to be specified in step 3, by running `$ cmake -G "MSYS Makefiles ~/ZipPicViewWx"`. 
+On Windows, [MSYS2](https://msys2.github.io/) is the preferred method. Install MSYS2 and necessary toolchain, then follow the same instruction as the UNIX toolchain. The Generator has to be specified in step 3, by running `$ cmake -G "MSYS Makefiles ~/ZipPicViewWx"`.
 
 After the program is built, `cpack` (which is part of CMAKE distribution) can be used to created installer. `ZIP` and `WIX` generators have been tested and used. To create installer package with `cpack`, run the command `$ cpack -G WIX` or `$ cpack -G ZIP` inside the build directory.
 
-To specify the version number into the build, pass `CPACK_PACKAGE_VERSION_MAJOR` and `CPACK_PACKAGE_VERSION_MAJOR` variable during step 3. For example `$ cmake -G "MSYS Makefiles" -DCPACK_PACKAGE_VERSION_MAJOR=0 -DCPACK_PACKAGE_VERSION_MINOR=7 ~/ZipPicViewWx`. By default the version is `DEV.0.0`
+To specify the version number into the build, pass `CPACK_PACKAGE_VERSION_MAJOR` and `CPACK_PACKAGE_VERSION_MAJOR` variable during step 3\. For example `$ cmake -G "MSYS Makefiles" -DCPACK_PACKAGE_VERSION_MAJOR=0 -DCPACK_PACKAGE_VERSION_MINOR=7 ~/ZipPicViewWx`. By default the version is `0.0.0`
 
 ZipPicViewWX depends on [wxWidgets](https://www.wxwidgets.org/) and [libzip](http://www.nih.at/libzip/). It has been compiled on GCC and Clang.
+
+### CImg Library
+
+An experimental [CImg](http://cimg.eu/) libary usage has been added. This enhance the image quality in the preview panel when the scale is not 100%. As a trade-off, the performance is degraded significantly. This functionality is currently disabled by default.
+
+To enable CImg usage, append the flag `-DUSE_CIMG=yes` as a cmake parameter.