Mention paper used for pupil tracking in comments

src/eye.cpp

@@ -1,5 +1,8 @@
 #include <eye.hpp>
 
+// This pupil tracking algorithm is based on the paper
+// "ACCURATE EYE CENTRE LOCALISATION BY MEANS OF GRADIENTS"
+// by Fabian Timm  and  Erhardt Barth. Thanks!
 
 float objectiveFunction(cv::Point2f c, cv::Mat gX, cv::Mat gY) {
 	float sum = 0;