Add Flatpak build instructions so I don't forget again

FLATPAK.md

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+# Flatpak Build Instructions
+
+This builds the Flatpak in a directory called `build` and a repository called `export`:
+
+```shell
+$ flatpak-builder --force-clean --user --install --repo=export build zone.xiv.astra.yml
+```
+
+Some other useful options I use:
+* `--disable-updates`: Disable updates for VCS like git, useful for quick iteration on Astra itself.
+
+To create a bundle, use this command:
+
+```shell
+$ flatpak build-bundle export astra.flatpak zone.xiv.astra  --runtime-repo=https://flathub.org/repo/flathub.flatpakrepo
+```
+
+This should create a bundle called `astra.flatpak`.