add minimal example

examples/minimal.py

@@ -0,0 +1,31 @@
+import essence
+
+class Position(essence.Component):
+    def __init__(self, x, y):
+        self.x = x
+        self.y = y
+
+class PhysicsSystem(essence.System):
+    def __init__(self, gravity):
+        self.gravity = gravity
+
+    def update(self, world):
+        for e in world.entites:
+            e.get(Position).y -= self.gravity
+
+class RenderSystem(essence.System):
+    def update(self, world):
+        for e in world.entites:
+            # Render entity...
+
+if __name__ == '__main__':
+    world = essence.World()
+    world.systems.append(PhysicsSystem(2))
+    world.systems.append(RenderSystem(2))
+
+    player = world.create_entity()
+    player.add(Position(10, 10))
+
+    while True:
+        world.update()
+