add decorator docs

docs/sanic/decorators.md

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+# Handler Decorators
+
+Since Sanic handlers are simple Python functions, you can apply decorators to them in a similar manner to Flask. A typical use case is when you want some code to run before a handler's code is executed. 
+
+## Authorization Decorator
+
+Let's say you want to check that a user is authorized to access a particular endpoint. You can create a decorator that wraps a handler function, checks a request if the client is authorized to access a resource, and sends the appropriate response.
+
+
+```python
+from functools import wraps
+from sanic.response import json
+
+def authorized():
+    def decorator(f):
+        @wraps(f)
+        async def decorated_function(request, *args, **kwargs):
+            # run some method that checks the request
+            # for the client's authorization status
+            is_authorized = check_request_for_authorization_status(request)
+
+            if is_authorized:
+                # the user is authorized.
+                # run the handler method and return the response
+                response = await f(request, *args, **kwargs)
+                return response
+            else:
+                # the user is not authorized. 
+                return json({'status': 'not_authorized'}, 403)
+        return decorated_function
+    return decorator
+
+
+@app.route("/")
+@authorized()
+async def test(request):
+    return json({status: 'authorized'})
+``` 
+