Create README with build instructions

README.md

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+# Pong Circle
+
+A remake of pong up to 4 players
+
+# Build
+
+You need `make` and `SDL1` to build this project.
+
+Example on Ubuntu :
+```
+# apt install make g++ libsdl-dev
+$ make
+```
+
+To execute :
+```
+$ make exec
+```