on_event deprecation

[waltzforvenus]

In the example Readme file @app.on_event("startup") is being used but at the moment it is being deprecated and getting replaced by lifespan functions.

Ideally the documentation should be updated to compliment the updates on the fastapi library itself.

For more info check:
https://fastapi.tiangolo.com/advanced/events/

[vicchi]

Resolved in the README.md as part of #426 so closing as wontfix.