[docs] Events: Explain "Add your event listener as late as possible"

[ghost]

### Where to add your event listeners

You need to add event listeners in a method that is guaranteed to fire before the event occurs. However, for optimal loading performance, you should add your event listener as late as possible.  

Expand on this, why is it ok to add event listeners in templates

@justinfagnani FYI

[kevinpschaaf]

The lit.dev docs for event listeners now include an Optimizing for Performance section which has some treatment on performance patterns.