Add document for custom protocol

docs/custom_protocol.md

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+# Custom Protocol
+
+You can change the behavior of protocol by using custom protocol.
+If you want to use custom protocol, you should put subclass of [protocol class](https://docs.python.org/3/library/asyncio-protocol.html#protocol-classes) in the protocol keyword argument of `sanic.run()`. The constructor of custom protocol class gets following keyword arguments from Sanic.
+
+* loop
+`loop` is an asyncio compatible event loop.
+
+* connections
+`connections` is a `set object`  to  store protocol objects.
+When Sanic receives `SIGINT` or `SIGTERM`, Sanic executes `protocol.close_if_idle()` for a `protocol objects` stored in connections.
+
+* signal
+`signal` is a `sanic.server.Signal object` with `stopped attribute`.
+When Sanic receives `SIGINT` or `SIGTERM`, `signal.stopped` becomes `True`.
+
+* request_handler
+`request_handler` is a coroutine that takes a `sanic.request.Request` object and a `response callback` as arguments.
+
+* error_handler
+`error_handler` is a `sanic.exceptions.Handler` object.
+
+* request_timeout
+`request_timeout` is seconds for timeout.
+
+* request_max_size
+`request_max_size` is bytes of max request size.
+
+## Example
+
+```python
+from sanic import Sanic
+from sanic.server import HttpProtocol
+from sanic.response import text
+
+app = Sanic(__name__)
+
+
+class CustomHttpProtocol(HttpProtocol):
+
+    def __init__(self, *, loop, request_handler, error_handler,
+                 signal, connections, request_timeout, request_max_size):
+        super().__init__(
+            loop=loop, request_handler=request_handler,
+            error_handler=error_handler, signal=signal,
+            connections=connections, request_timeout=request_timeout,
+            request_max_size=request_max_size)
+
+    def write_response(self, response):
+        if isinstance(response, str):
+            response = text(response)
+        self.transport.write(
+            response.output(self.request.version)
+        )
+        self.transport.close()
+
+
+@app.route('/')
+async def string(request):
+    return 'string'
+
+
+@app.route('/1')
+async def response(request):
+    return text('response')
+
+app.run(host='0.0.0.0', port=8000, protocol=CustomHttpProtocol)
+```