Add short instruction for how to build flatpak version

packaging/flatpak/README.md

+# Package Kdenlive as Flatpak
+
+## Stable Version
+The manifest for the stable Kdenlive version on Flathub is hosted on https://github.com/flathub/org.kde.kdenlive
+
+## Nightly Version
+The nightly flatpak is on the kde flatpak repository (`kdeapps`). The build scripts for this respository are living at https://invent.kde.org/packaging/flatpak-kde-applications but in Kdenlives case it is just a link to the `org.kde.kdenlive-nightly.json` file in this folder. It is build and published each day by the [KDE Binary Factory](https://binary-factory.kde.org/job/Kdenlive_x86_64_flatpak/)
+
+## How to build
+
+First you need to setup `flatpak` and [`flatpak-builder`](https://docs.flatpak.org/en/latest/flatpak-builder.html). On ubuntu you do it by running
+```
+sudo apt install flatpak flatpak-builder
+```
+
+To build Kdenlive you also need some dependencies:
+```
+flatpak install org.kde.Sdk
+```
+
+To build and install the Flatpak you can now switch to `packaging/flatpak` insider the Kdenlive source code folder and run:
+
+```
+flatpak-builder ~/flatpak-buildir org.kde.kdenlive-nightly.json --install
+```
+
+Read the flatpak build instructions for further general instructions on the flatpak build system: https://docs.flatpak.org/en/latest/building.html
+
+Tip: use `flatpak make-current master` or `flatpak make-current stable` to switch the current version for `flatpak run org.kde.kdenlive`