added README and testdata

README.md

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+# Sudoku verifier
+
+## Definition
+Sudokus are logic number puzzles in which cells in a 9×9 grid must be populated with 
+the digits 1 to 9 in a way that each digit occurrs only once in each unit (that is column, 
+row or block = 3×3 sub grid).
+
+![a solved simple sudoku](sudoku.gif)
+
+## Goal
+Validate if a given solution for a Sudoku is valid, by checking that the digit
+1 to 9 appear only once in column, in each row and in each of the 9 sub grids.
+
+### Input
+a sequence of 81 digits, organized in 9 columns and 9 rows
+
+### Output
+A boolean value, if the grid is valid.
+
+### Constraints
+1 <= `digit` <= 9
+
+### Examples
+**Input:** 
+```
+9 8 1 5 2 3 6 4 7
+6 3 4 8 7 9 2 5 1
+2 7 5 1 4 6 9 8 3
+1 9 6 4 8 7 5 3 2
+5 4 8 3 1 2 7 6 9
+7 2 3 6 9 5 4 1 8
+3 1 2 7 5 4 8 9 6
+4 6 9 2 3 8 1 7 5
+8 5 7 9 6 1 3 2 4
+```
+**Output:** 
+`true`