docs: update readme

README.md

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-# drop4
+# Drop4
 
-Puissance 4 written in C
+A simple implementation of the game Puissance 4, written in C.
+
+Why ? Because I got bored on friday night.
+
+## Implementation details
+
+The board is represented as a 2D array of integers.
+
+The last played piece is stored in the `last_row` and `last_col` variables, allowing us to efficiently check for a win by examining lines extending from that position in 4 directions (horizontal, vertical, and both diagonals).
+
+Game is a struct with the board, the current player, and the last played piece.
+
+What else... I guess input handling is prettty robust, I check most of the possible edge cases.
+
+## Rules
+
+The game is played on a 6x7 grid.
+
+The players take turns dropping their pieces into the grid.
+
+The first player to get 4 pieces in a row (horizontally, vertically, or diagonally) wins.
+
+## How to play
+
+Just compile and run the program.
+
+```bash
+make
+./drop4
+```
+
+## License
+
+BSD 3-Clause License