update README

README.md

 # Sudoku Solver
 
-This project aims at providing a Sudoku solver. The problem was given in the
-context of a technical assessment by a company.
+This project aims at providing a Sudoku solver.
 
 ## Problem
 
@@ -32,4 +31,15 @@ listing a few methods to solve a classic Sudoku.
 The simplest approach is backtracking, which is a brute-force search, meaning,
 we try all combinations of digits that fit until a rule is broken or until we
 reach the end of the puzzle. In the former case, we then try another value.
-If none is found, we update a previous digit. This is backtracking. 
+If none is found, we update a previous digit. This is backtracking.
+
+## Selected Algorithm and Rational behind it
+
+No performance requirement, in time or in space, were given. The same regarding
+hardware requirement. Having this freedom, I went for the simplest approach to
+solve this problem.
+
+On my machine, equipped with an AMD Ryzen 5 3600 @4.2GHz, solving the first
+problem takes between 12 and 13ms while, the problem 6, the large sudoku
+takes less than 0.5ms to solve (the grid is bigger, but the problem is much
+easier as the grid is almost full already).