Would like a TROUBLESHOOTING.md document

[reivilibre]

There's already too much to go wrong and too much to forget, so it'd be kind to our users (which are now growing in number and diversity of apps/environments) to have a troubleshooting document, because this stuff tends to go wrong without a clear idea of what went wrong.

Also, can document sample curl commands that people may find useful.

Things to mention so I don't forget:

• FCM for iOS and Android
• APNs — base64 encoding
• content-available (and maybe mutable-content) on iOS
  ...