Add raytracing in one weekend (#5362)

* Add raytracing in one weekend

* Add html to entry.

* Remove trailing slash

books/free-programming-books.md

@@ -296,6 +296,7 @@
 * [Learning Modern 3D Graphics Programming](https://web.archive.org/web/20150225192611/http://www.arcsynthesis.org/gltut/index.html) - Jason L. McKesson (draft)
 * [Notes for a Computer Graphics Programming Course](https://www.cs.csustan.edu/~rsc/CS3600F00/Notes.pdf) - Dr. Steve Cunningham (PDF)
 * [OpenGL](http://www.songho.ca/opengl/index.html) - Concepts and illustrations.
+* [Ray Tracing in One Weekend](https://raytracing.github.io) - Peter Shirley (HTML)
 * [ShaderX series](http://www.realtimerendering.com/resources/shaderx/) - Wolfgang Engel
 * [Tutorials for modern OpenGL](http://www.opengl-tutorial.org)
 * [WebGL Insights](http://webglinsights.com) - Patrick Cozzi and Contributors