docs: run a broker inside another framework's lifespan

[lesnik512]

Adds a README section for running a broker inside another framework's lifespan, such as FastAPI's. setup_di needs a FastStream app, so the broker goes in FastStream(broker) with manage_lifespan=False, and the lifespan calls start() and stop() inside async with container:. start() runs the on_startup hooks, where the middleware is installed.

The section also says that start() and stop() do not run a FastStream(lifespan=...), links the modern-di shared container recipe, and says FastStream's FastAPI routers need only modern-di-fastapi.

Checked against modern-di-faststream main with a NatsBroker under TestNatsBroker and FastAPI's TestClient: the subscriber and the endpoint resolve the same APP-scoped instance, and the finalizer runs once when the container closes.

Closes #79