Add lifespan docs

docs/en/docs/advanced/events.md

@@ -45,3 +45,15 @@ Here, the `shutdown` event handler function will write a text line `"Application
 
 !!! info
     You can read more about these event handlers in <a href="https://www.starlette.io/events/" class="external-link" target="_blank">Starlette's  Events' docs</a>.
+
+# lifespan
+
+You can also define a lifespan context as an async generator, instead of using startup and shutdown events.
+
+This is useful when the startup and shutdown events of your application are encompassed in a context manager.
+
+This function must be declared with `async def` and contain one `yield` statement.
+
+```Python hl_lines="8"
+{!../../../docs_src/events/tutorial003.py!}
+```

docs_src/events/tutorial003.py

@@ -0,0 +1,15 @@
+from fastapi import FastAPI
+
+
+async def lifespan(app):
+    print("startup")
+    yield
+    print("shutdown")
+
+
+app = FastAPI(lifespan=lifespan)
+
+
+@app.get("/")
+async def hello():
+    return {"result": "hello world"}