added description

README.md

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+ImageStreams
+============
+
+Motivation
+----------
+Some time ago I had a project where I rendered something to a high quality buffer, which I wanted to export as a png. I used libpng++ for that, the downside was that this library allocated the image again (in a different format). But the only thing I wanted to do is convert my format to another format (pixel by pixel), so this allocation seemed redundant. That's why I wanted a image stream library.
+
+Features
+--------
+* Output streams for bmp and png image files
+* Grayscale and color
+* Low memory usage (because it streams)
+* Easy to use
+
+Uasge
+-----
+```C++
+png::gray_ostream image(1024, 768, "test.png");
+for(int y = 0; y < 768; ++y){
+	for(int x = 0; x < 1024; ++x){
+		image << png::gray_ostream::pixel(some_generation(x,y));
+	}
+}
+// done, image is automatically saved (wtih RAII)
+```
+
+There are more examples in the main.cpp file.