WIP: Docs

docs/faq.md

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+# Frequently asked questions (FAQ)
+
+## Isn't this like ...?
+Who knows. I didn't do a lot of research before making this. It was fun making it.
+
+## Can I use this in my app? Will it stay free?
+Yes. As long as you don't abuse it, it'll be available and free of charge. I do not plan on monetizing
+the service.
+
+## What are the uptime guarantees?
+Best effort.
+
+## What happens if there are multiple subscribers to the same topic?
+As per usual with pub-sub, all subscribers receive notifications if they are
+subscribed to a topic.
+
+## Will you know what topics exist, can you spy on me?
+If you don't trust me or your messages are sensitive, run your own server. It's <a href="https://github.com/binwiederhier/ntfy">open source</a>.
+That said, the logs do not contain any topic names or other details about you.
+Messages are cached for the duration configured in `config.yml` (12h by default) to facilitate service restarts, message polling and to overcome
+client network disruptions.
+
+## Can I self-host it?
+Yes. The server (including this Web UI) can be self-hosted, and the Android app supports adding topics from
+your own server as well. There are <a href="https://github.com/binwiederhier/ntfy#installation">install instructions</a>
+on GitHub.
+
+## Why is Firebase used?
+In addition to caching messages locally and delivering them to long-polling subscribers, all messages are also
+published to Firebase Cloud Messaging (FCM) (if `FirebaseKeyFile` is set, which it is on ntfy.sh). This
+is to facilitate instant notifications on Android.
+    </p>
+
+## How much battery does the Android app use?
+If you use the ntfy.sh server and you don't use the <i>instant delivery</i> feature, the Android app uses no
+additional battery, since Firebase Cloud Messaging (FCM) is used. If you use your own server, or you use
+<i>instant delivery</i>, the app has to maintain a constant connection to the server, which consumes about 4% of
+battery in 17h of use (on my phone). I use it and it makes no difference to me.
+
+## What is instant delivery?
+Instant delivery is a feature in the Android app. If turned on, the app maintains a constant connection to the
+server and listens for incoming notifications. This consumes <a href="#battery-usage">additional battery</a>,
+but delivers notifications instantly.
+
+## Why is there no iOS app (yet)?
+I don't have an iPhone or a Mac, so I didn't make an iOS app yet. It'd be awesome if
+<a href="https://github.com/binwiederhier/ntfy/issues/4">someone else could help out</a>.