feat(permutations): add example to README for an empty array

Author: nik-rev

16_permutations/README.md

@@ -1,7 +1,8 @@
 # Exercise 16 - permutations
 
-Write a function that takes in an input array of an consecutive positive integers, starting at 1, and returns an array of all possible permutations of the original array
-
+Write a function that takes in an empty array or an input array of an consecutive positive integers, starting at 1, and returns an array of all possible permutations of the original array
 ```javascript
 permutations([1,2,3]); // [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]
+// An empty set has a single permutation, 0! = 1
+permutations([]); // [[]]
 ```