GUI: Add readme file with instructions for running and compiling the GUI.

gui/readme.txt

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+cppcheck GUI
+============
+This is a GUI for cppcheck. It allows selecting folder or set of files to check
+with cppcheck and shows list of found errors.
+
+Running
+-------
+You need QT4 libraries installed in your system. Packages/files to install
+depends on your operating system:
+- Windows download QT4 from http://www.qtsoftware.com
+- Linux install QT4 using your package manager, look for packages having QT4
+  in their name, e.g. for Ubuntu install libqt4-core and libqt4-gui
+
+Compiling
+---------
+Windows:
+- The easy way is to download Qt SDK from http://www.qtsoftware.com and use
+  Qt Creator and/or command line tools to build the GUI.
+- The harder way is to download QT sources and build QT with Visual Studio
+  (Express Edition works). Compiling QT alone may take over 4 hours!
+
+Linux:
+- Install QT development packages (make sure qmake -tool gets installed!). The
+  names depend on distribution, but e.g. for Ubuntu the needed packages are:
+  * libqt4-core
+  * libqt4-gui
+  * libqt4-dev
+  * qt4-dev-tools
+  * qt4-qmake
+
+After you have needed libraries and tools installed, open command
+prompt/console, go to gui directory and run command:
+- qmake (in Linux and in Windows if build with MinGW/gcc)
+- qmake -tp vc (In Windows if build with Visual Studio)
+
+These commands generate makefiles to actually build the software. After that
+the actual building is done in IDE or command line as usual. Note that you
+don't need to run qmake again unless you add/remove files from the project.