[Wiki] Add flatpak build docs

Buidling-the-flatpak.md

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+## Install dependencies
+
+Barrier is built against the [KDE runtime and SDK](https://github.com/flathub/com.github.debauchee.barrier/blob/master/com.github.debauchee.barrier.json#L3-L5),
+to install them, one should follow these steps:
+
+```shell
+$ flatpak --user remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
+$ flatpak --user install flathub org.kde.Platform/x86_64/3.28
+$ flatpak --user install flathub org.kde.Sdk/x86_64/3.28
+```
+You should replace `x86_64` with your architecture (`i386`, `arm`, `aarch64`, `x86_64`)
+and `3.28` with the current version of the runtimes and SDK used in the [`com.github.debauchee.barrier.json` file](https://github.com/flathub/com.github.debauchee.barrier/blob/master/com.github.debauchee.barrier.json#L3-L5).
+
+## Flatpak builder
+
+`flatpak-builder` can be installed as a flatpak, this provides the benefit of
+having a possibly more up-to-date version of `flatpak-builder`. To install,
+follow these steps:
+
+```shell
+$ flatpak --user install flathub org.flatpak.Builder
+```
+
+To make compilation smoother, we use `make`, which can be installed from your package manager.
+
+## Compiling the flatpak
+
+First thing you'll wan to do if you haven't already, is clone the repo:
+
+`git clone https://github.com/flathub/com.github.debauchee.barrier.git`
+
+Then `cd com.github.debauchee.barrier` and `make` or `make FLATPAK_BUILDER="flatpak run org.flatpak.Builder"`.
+
+### Cross compiling
+
+If you're on `x86_64` and want to
+compile for `i386`, this can be done via:
+
+`make BUILDER_OPTIONS="--arch=i386"`
+
+Keep in mind though, you'll need to install the `i386` platform and SDK.