Add NAppGUI (GUI library) (#1660)

Author: toge

README.md

@@ -598,6 +598,7 @@ A curated list of awesome C++ (or C) frameworks, libraries, resources, and shiny
 * [MyGUI](https://github.com/MyGUI/mygui) - Fast, flexible and simple GUI. [MIT]
 * [nana](http://nanapro.org/en-us/) - Nana is a cross-platform library for GUI programming in modern C++ style. [Boost]
 * [NanoGui](https://github.com/mitsuba-renderer/nanogui) - A minimalistic cross-platform widget library for OpenGL 3.x or higher. [BSD]
+* [NAppGUI](https://github.com/frang75/nappgui_src) - SDK for building cross-platform desktop apps in ANSI-C. [MIT] [website](https://nappgui.com/en/home/web/home.html)
 * [nuklear](https://github.com/Immediate-Mode-UI/Nuklear) - A single-header ANSI C gui library. [PublicDomain]
 * [QCustomPlot](http://qcustomplot.com/) - Qt plotting widget without further dependencies. [GPLv3]
 * [Qwt](http://qwt.sourceforge.net/) - Qt Widgets for Technical Applications. [Own based on LGPL]