Make simple readme with build instructions

Instructions work on Ubuntu 18.04

README.md

@@ -0,0 +1,18 @@
+# Rainbow Dodge
+
+Race a rainbow line and dodge rainbow blocs !
+
+# 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
+```