Added images, readme, slight fix

Readme.md

 # Fortuna mines
-WIP.
 Minesweeper for the LaFortuna. 
+It has 3 board sizes based on Microsofts Minesweeper:
+- Beginner: 9x9, 10 mines
+- Intermediate: 16x16, 20 mines
+- Expert: 30x16, 99 mines
+
+![Main menu](/images/main_menu.jpg)
+![Intermediate board](/images/medium_board.jpg)
+![Expert board](/images/expert_board.jpg)
+![Failed expert board, on mines screen](/images/mines_screen.jpg)
+
+## Installing
+You can either download the `mines.hex` file from releases and upload using:
+```shell
+dfu-programmer at90usb1286 erase
+dfu-programmer at90usb1286 flash mines.hex 
+```
+or compile and install using the provided makefile.
+
+## Playing
+
+Grey cells are undiscovered, white cells are empty, cells with numbers have n adjacent mines. Avoid detonating the mines whilst attempting to tag all mines or clear the board.
+Pink cells are tagged, yellow are questioned.
+
+### Movement
+- Uses rotary encoder to move either left/right or up/down
+- Use up to toggle between left/right or up/down movement
+- Left to tag, right button to question
+- Down pauses the game
+- Centre discovers and undiscovered cell, or sweeps if held down
 
 ## License breakdown
 - mines.c, mines.h - BSD 3-Clause

mines.c

@@ -459,7 +459,7 @@ void discover_pos(int pos){
 				adjacent_mines(pos,1);
 			}
 			cells_discovered++;
-			if(cells_discovered == (uint8_t) (BOARD_ITEMS - MAX_MINES)){
+			if(cells_discovered == ((uint16_t) BOARD_SIZE_X * (uint16_t) BOARD_SIZE_Y) - (uint16_t) MAX_MINES){
 				win();
 			}
 		}