Add a note about iOS Safari to README

[eggplants]

It is generally known that in iOS Safari, the browser's standard dialogs cannot be displayed using window.confirm or alert during the popstate event triggered by the browser back button.

turbolinks/turbolinks#336
https://am-yu.net/2022/01/26/ios-safari_dialog_not_working_browserback/

It would be helpful if the README included a note like, "To avoid this issue, please use the custom dialog component."