docs: add README

README.md

@@ -0,0 +1,15 @@
+# sudoku-solver
+
+Sudoku solver written in PHP.
+
+## Usage
+
+```bash
+php sudoku.php
+```
+
+Edit `sudoku.php` to define the puzzle (9×9 grid with `0` for empty cells).
+
+## License
+
+Apache-2.0