add websocket section in doc

docs/sanic/websocket.rst

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+WebSocket
+=========
+
+Sanic supports websockets, to setup a WebSocket:
+
+.. code:: python
+
+    from sanic import Sanic
+    from sanic.response import json
+    from sanic.websocket import WebSocketProtocol
+
+    app = Sanic()
+
+    @app.websocket('/feed')
+    async def feed(request, ws):
+        while True:
+            data = 'hello!'
+            print('Sending: ' + data)
+            await ws.send(data)
+            data = await ws.recv()
+            print('Received: ' + data)
+
+    if __name__ == "__main__":
+        app.run(host="0.0.0.0", port=8000, protocol=WebSocketProtocol)
+
+
+Alternatively, the ``app.add_websocket_route`` method can be used instead of the
+decorator:
+
+.. code:: python
+    async def feed(request, ws):
+        pass
+
+    app.add_websocket_route(feed, '/feed')
+
+
+Handlers for a WebSocket route are passed the request as first argument, and a
+WebSocket protocol object as second argument. The protocol object has ``send``
+and ``recv`` methods to send and receive data respectively.
+
+
+You could setup your own WebSocket configuration through ``app.config``, like
+
+.. code:: python
+    app.config.WEBSOCKET_MAX_SIZE = 2 ** 20
+    app.config.WEBSOCKET_MAX_QUEUE = 32
+    app.config.WEBSOCKET_READ_LIMIT = 2 ** 16
+    app.config.WEBSOCKET_WRITE_LIMIT = 2 ** 16
+
+Find more in ``Configuration`` section.