message: Add link to OpenGL in readme
diff: @@ -30,7 +30,7 @@ Build requirements
  - [CMake][1] version 2.8.9 or newer
  - [Boost][2]
  -- OpenGL
  +- [OpenGL][12]
  - [GLEW][3]
  - [NVIDIA CUDA Toolkit][4] version 7.0 or newer (optional)
  @@ -151,3 +151,4 @@ Visionaray is licensed under the MIT License (MIT)
  [9]: http://www.libtiff.org
  [10]: http://www.openexr.com/
  [11]: https://github.com/ingowald/pbrt-parser
  +[12]: https://www.opengl.org
  