added README.md

README.md

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+## Controls
+- press **space bar** to start the game
+- use **mouse scroll** to zoom in/out
+- keep pressed **mouse right button** to pan
+- use **mouse left click** to switch on/off individual pixels
+- press `[` to decrease game speed and `]` to increase it
+- press `c` to clear the screen
+- press `s` to do a single game step
+- press `h` to restore the default zoom level
+
+## Demo
+You can find a live example [here](http://woggioni.net/game_of_life) (requires a keyboard and a mouse)
+
+## Build
+to compile for desktop
+
+```
+nim c -d:release -o:sdlife main.nim
+```
+
+to compile for web browsers using [emscripten](https://emscripten.org/)
+
+```
+nim c -d:release -d:wasm main.nim
+```
+