README: Add note about Coursera Discrete Math (paid)

Author: joshmhanson

README.md

@@ -195,7 +195,10 @@ Courses | Duration | Effort | Prerequisites
 [Calculus 1A: Differentiation](https://www.edx.org/course/calculus-1a-differentiation-mitx-18-01-1x-0) | 13 weeks | 6-10 hours/week | pre-calculus
 [Calculus 1B: Integration](https://www.edx.org/course/calculus-1b-integration-mitx-18-01-2x-0) | 13 weeks | 5-10 hours/week | Calculus 1A
 [Calculus 1C: Coordinate Systems & Infinite Series](https://www.edx.org/course/calculus-1c-coordinate-systems-infinite-mitx-18-01-3x-0) | 13 weeks | 5-10 hours/week | Calculus 1B
-[Mathematics for Computer Science](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-042j-mathematics-for-computer-science-spring-2015/index.htm) | 13 weeks | 5 hours/week | Calculus 1C
+[Mathematics for Computer Science](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-042j-mathematics-for-computer-science-spring-2015/index.htm)<sup>**1**</sup> | 13 weeks | 5 hours/week | Calculus 1C
+
+<sup>**1**</sup>: Students struggling with MIT Math for CS can consider taking the [Discrete Mathematics Specialization](https://www.coursera.org/specializations/discrete-mathematics) first.
+It is more interactive but less comprehensive, and it costs money to unlock full interactivity.
 
 ### Core systems