✨ Add docs for GraphQL (#48)

docs/tutorial/graphq.md

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+
+**FastAPI** has optional support for GraphQL (provided by Starlette directly), using the `graphene` library.
+
+You can combine normal FastAPI path operations with GraphQL on the same application.
+
+## Import and use `graphene`
+
+GraphQL is implemented with Graphene, you can check <a href="https://docs.graphene-python.org/en/latest/quickstart/" target="_blank">Graphene's docs</a> for more details.
+
+Import `graphene` and define your GraphQL data:
+
+```Python hl_lines="1 6 7 8 9 10"
+{!./src/graphql/tutorial001.py!}
+```
+
+## Add Starlette's `GraphQLApp`
+
+Then import and add Starlette's `GraphQLApp`:
+
+```Python hl_lines="3 14"
+{!./src/graphql/tutorial001.py!}
+```
+
+!!! info
+    Here we are using `.add_route`, that is the way to add a route in Starlette (inherited by FastAPI) without declaring the specific operation (as would be with `.get()`, `.post()`, etc).
+
+## Check it
+
+Run it with Uvicorn and open your browser at <a href="http://127.0.0.1:8000" target="_blank">http://127.0.0.1:8000</a>.
+
+You will see GraphiQL web user interface:
+
+<img src="/img/tutorial/graphql/image01.png">
+
+
+## More details
+
+For more details, including:
+
+* Accessing request information
+* Adding background tasks
+* Using normal or async functions
+
+check the official <a href="https://www.starlette.io/graphql/" target="_blank">Starlette GraphQL docs</a>.