📝 Add docs for middleware (#173)

docs/tutorial/middleware.md

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+You can add middleware to **FastAPI** applications.
+
+A "middleware" is a function that works with every **request** before it is processed by any specific *path operation*. And also with every **response** before returning it.
+
+* It takes each **request** that comes to your application.
+* It can then do something to that **request** or run any needed code.
+* Then it passes the **request** to be processed by the rest of the application (by some *path operation*).
+* It then takes the **response** generated by the application (by some *path operation*).
+* It can do something to that **response** or run any needed code.
+* Then it returns the **response**.
+
+## Create a middleware
+
+To create a middleware you use the decorator `@app.middleware("http")` on top of a function.
+
+The middleware function receives:
+
+* The `request`.
+* A function `call_next` that will receive the `request` as a parameter.
+    * This function will pass the `request` to the corresponding *path operation*.
+    * Then it returns the `response` generated by the corresponding *path operation*.
+* You can then modify further the `response` before returning it.
+
+```Python hl_lines="9 10 12 15"
+{!./src/middleware/tutorial001.py!}
+```
+
+!!! tip
+    This technique is used in the tutorial about <a href="https://fastapi.tiangolo.com/tutorial/sql-databases/" target="_blank">SQL (Relational) Databases</a>.
+
+### Before and after the `response`
+
+You can add code to be run with the `request`,  before any *path operation* receives it. 
+
+And also after the `response` is generated, before returning it.
+
+For example, you could add a custom header `X-Process-Time` containing the time in seconds that it took to process the request and generate a response:
+
+```Python hl_lines="11 13 14"
+{!./src/middleware/tutorial001.py!}
+```
+
+## Starlette's Middleware
+
+You can also add any other <a href="https://www.starlette.io/middleware/" target="_blank">Starlette Middleware</a>.
+
+These are classes instead of plain functions.
+
+Including:
+
+* `CORSMiddleware` (described in the next section).
+* `GZipMiddleware`.
+* `SentryMiddleware`.
+* ...and others.