Update README.md for iOS Web Push and VAPID credentials

[daronspence]

Since Apple added web push support to iOS in 2023, there hasn't been a lot of details around what exactly is required.

Following the readme in this project, push notifications were working for Chromium browsers.

Apple follows a part of the spec that requires a VAPID subject to generate the push JWT. I found this specific information through a Java based library elsewhere on Github. https://github.com/web-push-libs/webpush-java/issues/201#issuecomment-1443258546

Adding this information should make things a lot clearer for people trying to add native web push support to their web apps moving forward.