started on documentation

README.md

@@ -11,6 +11,20 @@ Viz2D is a new way of writing graphical (on- and offscreen) applications with Op
 * Video pipeline: Through a simple Source/Sink system videos can be displayed, edited and saved.
 * Hardware acceleration: Automatic hardware acceleration usage where possible. (e.g. cl-gl sharing and VAAPI). Actually it is possible to write programs to run almost entirely on the GPU, given driver-features are available.
 
+# Documentation
+
+## Basics
+* Viz2D is not thread safe. Though it is possible to have several Viz2D objects in one or more threads and synchronize them using ```Viz2D::makeNonCurrent()``` and ```Viz2D::makeCurrent()```. This is a limitation of GLFW3.
+* Access to different subsystems (opengl, opencl, nanovg and nanogui) is provided through "contexts". A context is simply a function that takes a functor, sets up the subsystem, executes the functor and tears-down the subsystem.
+
+For example, to create an OpenGL context and set the GL viewport:
+```C++
+v2d->gl([](const cv::Size sz) {
+    glViewPort(0, 0, sz.width, sz.height);
+});
+```
+* Viz2D uses InputArray/OutputArray/InputOutputArray which gives you the option to work with cv::Mat, std::vector and cv::UMat. Anyway, you should prefer to use cv::UMat whenever possible to automatically use hardware capabilities where available.
+
 # Attribution
 * The author of the bunny video is **(c) copyright Blender Foundation | www.bigbuckbunny.org**.
 * The author of the dance video is **GNI Dance Company** ([Original video](https://www.youtube.com/watch?v=yg6LZtNeO_8))