add a few more references

glsl/mandelbulb.frag

 * Adapted for Fragmentarium from [1].
 *
 * [1]: http://2008.sub.blue/blog/2009/12/13/mandelbulb.html
 * [2]: http://www.fractalforums.com/theory/summary-of-3d-mandelbrot-set-formulas/
 * [3]: http://blog.hvidtfeldts.net/index.php/2011/06/distance-estimated-3d-fractals-part-i/
 * [4]: http://9bitscience.blogspot.co.uk/2013/07/raymarching-distance-fields_14.html
 */
#include "3D.frag"