Added book "Graph Theory Exercises" (#9268)

Added to Mathematics for Computer Science section.

books/free-programming-books-subjects.md

@@ -548,6 +548,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Combinatorial Problems And Exercises (1979)](https://archive.org/details/in.ernet.dli.2015.141538/) - L. Lovasz *(:card_file_box: archived)*
 * [Discrete Structures for Computer Science: Counting, Recursion, and Probability](http://cglab.ca/~michiel/DiscreteStructures/) - Michiel Smid
 * [Exploring Math for Programmers and Data Scientists](https://freecontent.manning.com/free-ebook-exploring-math-for-programmers-and-data-scientists/) - Paul Orland
+* [Graph Theory Exercises](https://www.ime.usp.br/~pf/graph-exercises/) - Paulo Feofiloff (PDF)
 * [Isomorphism -- Mathematics of Programming](https://github.com/liuxinyu95/unplugged) - Larry LIU Xinyu
 * [Mathematics for Computer Science](https://courses.csail.mit.edu/6.042/spring18/mcs.pdf) - Eric Lehman, F. Thomson Leighton, Albert R. Meyer (PDF)