Create README

With build instructions.

README.md

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+# Hyper Gunners
+
+A shooting game where can pick and meld fire guns!
+
+# Build
+
+You need `make` and `SDL1` to build this project.
+
+Example on Ubuntu :
+```sh
+$ sudo apt install make g++ libsdl-dev libsdl-gfx1.2-dev
+$ make
+```
+
+To execute :
+```sh
+$ make exec
+```