docs: document app.plugin.js in package.json exports

[vonovak]

Summary

As people start using package.json exports more, it might be worth highlighting the "usual" exports people might want to have, which includes expo config plugin files (app.plugin.js).

Feel free to close if you see this being out of scope.

Thank you :)

Test plan

[satya164]

Thanks for the PR. I wouldn't add it here since these is the generic setup that people can copy/paste.

Let's add it as another paragraph below "The ./package.json field is used to..." to say something like, "Make sure to also add any other files that other tools may read, for example ./app.plugin.js if you provide a Expo Config plugin.

Then below that you can show code snippet for "./app.plugin.js": "./app.plugin.js" in the diff syntax.