Add build instructions

README.md

@@ -1,3 +1,18 @@
 # Fire
 
-My first SDL project where you have to shoot down incoming copters !
+My first SDL project where you have to shoot down incoming copters !
+
+## Build
+
+You need `make` and `SDL1` to build this project.
+
+Example on Ubuntu :
+```sh
+$ sudo apt install make g++ libsdl-dev
+$ make
+```
+
+To execute :
+```sh
+$ make exec
+```