Added course called "Discrete optimization"

Very useful and interesting course with challengeable tasks and fantastic instructor.

Author: QtRoS

extras/courses.md

@@ -55,6 +55,7 @@ Courses | Duration | Effort
 Courses | Duration | Effort
 :-- | :--: | :--:
 [Analysis of Algorithms](https://www.coursera.org/course/aofa)| 6 weeks | 6-8 hours/week
+[Discrete Optimization](https://www.coursera.org/course/optimization)| 9 weeks of study | 10-20 hours/week
 
 ### Programming Paradigms