Notifications docs

[dkrasner]

Pull Request Checklist

• [x] I read the contributing guide
• [ ] UI change has been tested on both light and dark themes, in portrait and landscape orientations and on iPhone and iPad simulators
• [ ] Accessibility has been taken into account.

• [x] Pull request is based on the develop branch

• [ ] Pull request contains a changelog file in ./changelog.d
• [x] You've made a self review of your PR
• [ ] Pull request includes screenshots or videos of UI changes
• [ ] Pull request includes a sign off

Main points

This PR adds a CONFIGURATION.md file and instructions/notes on setting up push notifications (APNs) for the app. It closes #5735

[pixlwave]

Hey @dkrasner, thank you so much for taking the time to add this documentation 🙌 Looks great to me.

I've added a couple of comments/suggestions inline. Let me know what you think and then we'll get this merged 👍

[pixlwave]

Oh, and for the PR to be merged please could you add

• a sign off
• a changelog file in ./changelog.d (called 5735.doc).

[CLAassistant]

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