Merge pull request #49 from bernardwolff/master

added graphics directory with two papers

computer_graphics/README.md

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+* [the-rendering-equation](http://x86.cs.duke.edu/courses/cps124/fall09/notes/16_rendering/p143-kajiya.pdf)
+* [an-improved-illumination-model-for-shaded-display](https://www.cs.drexel.edu/~david/Classes/CS586/Papers/p343-whitted.pdf)