Update readme.md to include how to run the game

readme.md

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 A multiplayer arcade style game implemented using a GUI interface with JavaFX.\
 Grade: 1st
 
-[Walkthrough video](https://youtu.be/mGMy9KwHcZQ)
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+[Walkthrough video](https://youtu.be/mGMy9KwHcZQ)
+
+---
+## Usage
+To run the game, enter the following command in a terminal when in the root directory:
+```
+mvn javafx:run
+```
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