Create 21_Combinations.cs

21. Write a program that reads two numbers N and K and generates all the combinations of K distinct elements from the set [1..N].
    Example:	N = 5, K = 2 -> {1, 2}, {1, 3}, {1, 4}, {1, 5}, {2, 3}, {2, 4}, {2, 5}, {3, 4}, {3, 5}, {4, 5}