Add build instructions

and title

README.md

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+# Bullet Auto Pilots
+
+A Box2D sandbox where you try to escape bullets shot by several artillery.
+
+# Build
+
+You need `make`, `SDL1` and [Box2D](https://github.com/erincatto/box2d) to build this project.
+
+Example on Ubuntu :
+```sh
+$ sudo apt install make g++ libsdl-dev libsdl-gfx1.2-dev libbox2d-dev
+$ make
+```
+
+**Warning about Box2D** : you need at least version 2.4.1 in order to compile this project. If your package manager is out of date, please build Box2D and install it from source.
+
+To execute :
+```sh
+$ make exec
+```