feat: add flatpak builds (#1230)

docs/flatpak.md

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+# Flatpak distribution
+
+The `.flatpak` binary files (known as "bundles") that
+`./gradlew packageInstallers` creates can be used to download and install
+Haveno, but there are several security issues that arise in Flatpak when only
+using the bundle files:
+
+- There is no
+[digital signature](https://en.wikipedia.org/wiki/Digital_signature),
+if a bad actor were to upload a malicious `.flatpak` the users would have no
+way to tell when upgrading.
+- Upgrading isn't as easy, your users need to find the new Flatpak bundle file,
+and you cannot update multiple apps easily.
+  - This also makes an accidental downgrade much more likely.
+
+Flatpak has a solution for these issues, a
+[Flatpak repository](https://docs.flatpak.org/en/latest/repositories.html).
+Flatpak repos store the data of their apps within an OSTree (almost like git)
+repository, and the commits can be signed with a GPG key. The nature of OSTree
+also allows for easy updates, as the Flatpak client can download deltas of the
+changes instead of the entire file.
+
+If you plan on distributing Haveno as a Flatpak, it's recommended to create a
+Flatpak repository as well. This guide will show you how to create a Flatpak
+repository for Haveno. The official documentation states that [it's possible to
+use GitHub/Lab Pages](https://docs.flatpak.org/en/latest/hosting-a-repository.html#hosting-a-repository-on-gitlab-github-pages)
+to host the repository, but this hasn't been tested. The more common way is to
+use a web server, or something like
+[flat-manager](https://github.com/flatpak/flat-manager).
+
+An example Haveno flat-manager solution using `docker-compose` has been created
+and documented at <https://github.com/haveno-dex/flatman-haveno-test> if you
+want a quick way to get started. Note that this does require an always-on server.