✨ Add docs for WebSockets (#62)

docs/tutorial/websockets.md

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+
+You can use <a href="https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API" target="_blank">WebSockets</a> with **FastAPI**.
+
+## WebSockets client
+
+### In production
+
+In your production system, you probably have a frontend created with a modern framework like React, Vue.js or Angular.
+
+And to communicate using WebSockets with your backend you would probably use your frontend's utilities.
+
+Or you might have a native mobile application that communicates with your WebSocket backend directly, in native code.
+
+Or you might have any other way to communicate with the WebSocket endpoint.
+
+---
+
+But for this example, we'll use a very simple HTML document with some JavaScript, all inside a long string.
+
+This, of course, is not optimal and you wouldn't use it for production.
+
+In production you would have one of the options above.
+
+But it's the simplest way to focus on the server-side of WebSockets and have a working example:
+
+```Python hl_lines="2 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 42 43 44"
+{!./src/websockets/tutorial001.py!}
+```
+
+## Create a `websocket_route`
+
+In your **FastAPI** application, create a `websocket_route`:
+
+```Python hl_lines="3 47 48"
+{!./src/websockets/tutorial001.py!}
+```
+
+!!! tip
+    In this example we are importing `WebSocket` from `starlette.websockets` to use it in the type declaration in the WebSocket route function.
+
+    That is not required, but it's recommended as it will provide you completion and checks inside the function.
+
+
+!!! info
+    This `websocket_route` we are using comes directly from <a href="https://www.starlette.io/applications/" target="_blank">Starlette</a>. 
+    
+    That's why the naming convention is not the same as with other API path operations (`get`, `post`, etc).
+
+
+## Await for messages and send messages
+
+In your WebSocket route you can `await` for messages and send messages.
+
+```Python hl_lines="49 50 51 52 53"
+{!./src/websockets/tutorial001.py!}
+```
+
+You can receive and send binary, text, and JSON data.
+
+To learn more about the options, check Starlette's documentation for:
+
+* <a href="https://www.starlette.io/applications/" target="_blank">Applications (`websocket_route`)</a>.
+* <a href="https://www.starlette.io/websockets/" target="_blank">The `WebSocket` class</a>.
+* <a href="https://www.starlette.io/endpoints/#websocketendpoint" target="_blank">Class-based WebSocket handling</a>.
+
+
+## Test it
+
+If your file is named `main.py`, run your application with:
+
+```bash
+uvicorn main:app --debug
+```
+
+Open your browser at <a href="http://127.0.0.1:8000" target="_blank">http://127.0.0.1:8000</a>.
+
+You will see a simple page like:
+
+<img src="/img/tutorial/websockets/image01.png">
+
+You can type messages in the input box, and send them:
+
+<img src="/img/tutorial/websockets/image02.png">
+
+And your **FastAPI** application with WebSockets will respond back:
+
+<img src="/img/tutorial/websockets/image03.png">
+
+You can send (and receive) many messages:
+
+<img src="/img/tutorial/websockets/image04.png">
+
+And all of them will use the same WebSocket connection.