WIP basic flatpak for flathub instructions

docs/FLATPAK.md

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+# Configuring for Flatpak and Flathub
+
+Fluentflame Reader seeks to have regularly built and uploaded Flathub
+distributions. For those wanting to go through the process of Creating a
+Flathub-compatible flatpak build of Fluentflame Reader, this document is
+for you.
+
+## Dependencies
+
+You need to have `flatpak` itself installed. Install it via your Linux
+system package manager. e.g. for Debian:
+
+```bash
+# Debian based distros (e.g. Debian, Ubuntu, Mint, Devuan)
+sudo apt install flatpak
+```
+
+Add the flathub remote:
+
+```bash
+flatpak remote-add --if-not-exists --user \
+  flathub https://dl.flathub.org/repo/flathub.flatpakrepo
+```
+
+Install the `flatpak-builder` flatpak, and various SDK dependencies:
+
+```bash
+# Builder
+flatpak install -y flathub org.flatpak.Builder
+
+# Dependencies we use for the runtime
+flatpak install -y flathub
+```
+
+## Getting the source code
+
+TODO
+
+## Getting `flatpak/generated-sources.json`
+
+We have to generate a set of source files for all NPM dependencies.
+This source file must exactly match the `patches-lock.json` file at
+the given commit.
+
+### Cleanup
+
+The generation script will NOT work if you have an existing `node_modules`
+directory.
+
+```bash
+# Inside fluentflame-reader
+rm -rf node_modules/
+```
+
+You also need to be 100% certain that your `package-lock.json` matches the
+exact version of the archive we'll be pulling from the manifest. This may
+mean you need to change the manifest file or you need to replace your
+package-lock.json to match the archive you're pulling down.
+
+If a build has failed midway through, you may want to delete these
+intermediate directories:
+
+```
+# Inside fluentflame-reader, after a failed build
+rm -rf flatpak_out .flatpack-builder
+```
+
+This will get you in a fresh build state.
+
+### Generating the node dependency sources
+
+This is easiest to use with `pipx` as directed by the [documentation](https://github.com/flatpak/flatpak-builder-tools/tree/master/node).
+Install it like so through `pipx`:
+
+```bash
+pipx install git+https://github.com/flatpak/flatpak-builder-tools.git#subdirectory=node
+```
+
+Otherwise, install it [from pypi](https://pypi.org/project/flatpak-node-generator/)
+using your method of choice (e.g. pip venvs).
+
+Then run the following:
+
+```bash
+mkdir -p flatpak
+flatpak-node-generator -o flatpak/generated-sources.json npm package-lock.json
+```
+
+This will record the lockfile sha's into `flatpak/generated-sources.json`,
+which will direct flatpak builder on the exact dependency versions we need.
+
+## Building the Flatpak
+
+Now that everything is in order, we should now be able to build the flatpak
+from manifest:
+
+```bash
+flatpak run org.flatpak.Builder \
+  flatpak_out \
+  org.fluentflame.fluentflame-reader.yaml
+```
+
+This should then produce the final Flathub-compatible flatpak in `flatpak_out`.