add lesson 06

1. Link to
http://lazyfoo.net/tutorials/SDL/06_extension_libraries_and_loading_other_image_formats/index.php

2. You may get a runtime error about zlib1.dll. You should seek
zlib1.dll in your `path`, and check if it's version lower than 1.2.8. If
lower, you should copy the zlib1.dll which located same directory with
sdl2_image, to overwrite the lower one.