Problem 4: complete rewrite of the solution. (6 s -> 20 ms)

The solution as at least 6 digits, because 101101 = 143*707,
so we can just iter over the first 3 digits, create a palindrome
by appending their mirror, and check if the number can be decomposed
in two 3 digits factors.
README: Remove problems explanation (use comments or problem specific README), add 2 FAQs.