EASY_SHERLOCK_&_GCD: Added Documentation

Re-read the program I submitted at Hackerrank, for a gcd problem in
Downey, and added some documentation
GCD- Euclid's Algorithm (Downey's Think Python)

This program is a solution to a problem in Downey. This code has two
type of functions: iterative & recursive. A recursive code is one that
calls itself.