Update README

Make it more evident that the lib uses OpenCV drawing primitives instead of OpenGL or Qt.

README.md

@@ -1,13 +1,14 @@
 cvui
 =====
-A (very) simple UI lib built on top of OpenCV drawing primitives. Sometimes you don't have (or don't want) to use OpenGL and/or Qt in your OpenCV application!
+A (very) simple UI lib built on top of OpenCV drawing primitives. Other UI libs, such as [imgui](https://github.com/ocornut/imgui), require a graphical backend (e.g. OpenGL) to work, so if you want to use imgui in a OpenCV app, you must make it OpenGL enabled, for instance. It is not the case with cvui, which uses *only* OpenCV drawing primitives to do all the rendering (no OpenGL or Qt required).
 
 ![image](https://raw.githubusercontent.com/Dovyski/depository/master/cvui.png)
 
 Features
 --------
 - Lightweight and simple to use user interface.
 - No external dependencies (except OpenCV).
+- Based on OpenCV drawing primitives only (OpenGL or Qt are not required).
 - Friendly and C-like API (no classes/objects, etc).
 - MIT licensed.