Add link to new exercises

Author: tshemsedinov

Exercises.ru.md

@@ -14,3 +14,4 @@
 5. [Замыкания и чеининг](https://github.com/HowProgrammingWorks/Closure/blob/master/Exercises.ru.md)
 6. [Композиция функций](https://github.com/HowProgrammingWorks/Composition/blob/master/Exercises.ru.md)
 7. [Работа с массивами](https://github.com/HowProgrammingWorks/Arrays/blob/master/Exercises.ru.md)
+8. [Функции высшего порядка](https://github.com/HowProgrammingWorks/HigherOrderFunction/blob/master/Exercises.ru.md)