docs(readme): add algorithm, input format, and example

README.md

@@ -1,6 +1,35 @@
 # sudoku-solver
 
-Sudoku solver written in PHP.
+9×9 Sudoku solver in PHP using recursive backtracking — runs from the browser or CLI, prints solution to stdout.
+
+## Algorithm
+
+Recursive backtracking: iterates through empty cells, tries digits 1–9, and backtracks on constraint violations (row, column, and 3×3 box checks).
+
+## Input format
+
+Edit `sudoku.php` to define the puzzle as an 81-character string — digits 1–9 for known cells, `0` for empty cells:
+
+```php
+$puzzle = new SudokuSolver("530070000600195000098000060800060003400803001700020006060000280000419005000080079");
+$puzzle->solve();
+```
+
+## Example
+
+Input:
+```
+5 3 0 | 0 7 0 | 0 0 0
+6 0 0 | 1 9 5 | 0 0 0
+0 9 8 | 0 0 0 | 0 6 0
+```
+
+Output (stdout):
+```
+5 3 4 | 6 7 8 | 9 1 2
+6 7 2 | 1 9 5 | 3 4 8
+1 9 8 | 3 4 2 | 5 6 7
+```
 
 ## Usage
 
@@ -8,8 +37,8 @@ Sudoku solver written in PHP.
 php sudoku.php
 ```
 
-Edit `sudoku.php` to define the puzzle (9×9 grid with `0` for empty cells).
+Or open `sudoku.php` in a browser for the interactive grid UI.
 
 ## License
 
-Apache-2.0
+Apache-2.0 — see [LICENSE](LICENSE).