Add Callback functions

Callback-functions.md

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+You can pass the following functions to s.start() in a dictionary:
+|Name|Arguments passed to function|Timing|
+|---|---|---|
+|"init"|None|Once when the engine starts|
+|"update"|None|60 times per second|
+|"draw"|None|60 times per second, after update|
+|"resized"|width, height|Whenever the canvas changes size|
+|"tap"|x, y, id|When a click or touch begins|
+|"untap"|x, y, id|When a click is released or a touch ends|
+|"tapping"|x, y, id|Every frame while a pointer is held down and moving|
+|"tapped"|x, y, id|Triggered when a quick click or tap is completed|
+
+## Example:
+```python
+import screen as s
+
+def init():
+    pass
+
+def update():
+    pass
+    
+def draw():
+    pass
+    
+def resized(w, h):
+    pass
+    
+def tap(x, y, id):
+    pass
+
+def untap(x, y, id):
+    pass
+
+def tapping(x, y, id):
+    pass
+
+def tapped(x, y, id):
+    pass
+
+s.start({
+    "loop": {
+        "init": init,
+        "update": update,
+        "draw": draw,
+        "resized": resized,
+        "tap": tap,
+        "untap": untap,
+        "tapping": tapping,
+        "tapped": tapped
+    }
+})
+```