Add notes of caution about `sideEffects` key in `package.json`

[KonnorRogers]

Hi there! I stumbled on your guide today on Bluesky and noticed the note about "sideEffects". While it currently says its "needed", im here to provide some feedback on how sideEffects has burned me. Feel free to close, but heres a blog post I made with some links to various broken packages as a result of using the sideEffects key.

https://www.konnorrogers.com/posts/2023/the-shoelace-side-effect-scavenger-hunt

( you can ignore everything after the linked github issues)