Add readme and screenshots

README.md

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+# Flipper Zero Quadrastic Game
+
+**Quadrastic** is a simple addicting game inspired by the [Arduboy game](https://community.arduboy.com/t/quadrastic-secure-the-point-avoid-the-enemies) of the same name. This, in turn, was based on a popular PSP homebrew game called [Squarez](https://www.gamebrew.org/wiki/Squarez_PSP). Squarez itself drew inspiration from Ball, a game created by Nicolay Edin and Axel Hammarback, which was originally inspired by Eskiv, a game developed by Jean-François Geyelin.
+
+## Screenshots
+
+![screenshot-1](screenshots/screenshot-1.png)
+![screenshot-2](screenshots/screenshot-2.png)
+![screenshot-3](screenshots/screenshot-3.png)
+![screenshot-4](screenshots/screenshot-4.png)
+![screenshot-5](screenshots/screenshot-5.png)
+
+## Building the game
+
+### Cloning source code
+
+Make sure you clone the source code with submodules:
+
+``` sh
+git clone https://github.com/ivanbarsukov/flipperzero-quadrastic.git --recurse-submodules
+```
+
+### Building
+
+Build the game using [micro Flipper Build Tool](https://pypi.org/project/ufbt/):
+
+``` sh
+ufbt
+```
+
+You can now run the application (actually, the build step is not needed, it will be built and then launched):
+
+``` sh
+ufbt launch
+```