[programming/events] include the need to register your system

[Zarthus]

In https://bevy-cheatbook.github.io/programming/events.html, it was unclear to me that I also needed to trigger the event (e.g. using an Update or FixedUpdate), my silly assumption was that Bevy would automatically know if the EventReader<T> has been filled if an EventWriter<T> sends one and that it doesn't need periodic running. (and that it was handled with app.add_event::<>(); and the reading functions somehow magically discovered.)

For it to click for me, I had to refer to the official example (https://github.com/bevyengine/bevy/blob/v0.13.0/examples/ecs/event.rs) which registers the function too as a system.

While it is written in the "usage advice", it kind of flew over my head.

I'd suggest a simple enhancement here

bevy-cheatbook/src/programming/events.md

Lines 29 to 33 in fdf50e4

	You need to register your custom event types via the [app builder][cb::app]:
	```rust,no_run,noplayground
	{{#include ../code013/src/programming/events.rs:events-appbuilder}}
	```

to the sorts of something like this:

As well as register the system

app.add_systems(Update, debug_levelups);

I love the book, it's been super helpful :) this is the only pitfall I fell into so far.