Add explanation for the Binary Exponentiation

Maths/BinaryExponentiationIterative.js

@@ -1,6 +1,8 @@
 // To calculate x^n i.e. exponent(x, n) in O(log n) time in iterative way
 // n is an integer and n >= 0
 
+// Explanation: https://en.wikipedia.org/wiki/Exponentiation_by_squaring
+
 // Examples:
 // 2^3 = 8
 // 5^0 = 1