Add Flatpak build instructions to CONTRIBUTION.md (#279)

CONTRIBUTING.md

@@ -7,7 +7,48 @@ All contributions that improve Vorta for everyone are welcome. Before coding a n
 
 ## Local Development Setup
 
-Clone the latest version of this repo
+### Linux
+
+Follow the setup guide on [flatpak.org](http://flatpak.org/setup/) to make sure you have ``flatpak`` and ``flathub`` installed.
+
+You also need to have ``flatpak-builder`` installed, which is usually available from the same repository as the ``flatpak`` package.
+
+Install the org.kde 5.12 runtime and SDK
+```
+$ flatpak install flathub org.kde.Platform//5.12 org.kde.Sdk//5.12
+```
+
+Make a repo for your local builds (If you get the error: ``Remote listing for my-flatpak-builds not available; server has no summary file. Check the URL passed to remote-add was valid`` you can ignore it):
+```
+$ mkdir ~/my-flatpak-builds
+$ flatpak remote-add --user --no-gpg-verify my-flatpak-builds ~/my-flatpak-builds
+
+```
+
+Clone the latest version of this repo:
+```
+$ git clone https://github.com/borgbase/vorta/
+```
+
+Next, in Vorta’s source directory, use ``flatpak-builder`` to build a Vorta flatpak and install it (this can take more than an hour):
+```
+$ cd vorta/flatpak/
+$ mkdir app
+$ flatpak-builder --repo=$HOME/my-flatpak-builds app com.borgbase.vorta.yaml
+$ flatpak install --user my-flatpak-builds com.borgbase.vorta -y
+```
+
+Now you can work on the codebase.
+When you want to test your changes you can execute inside of ``vorta/flatpak/``
+```
+$ flatpak-builder --repo=$HOME/my-flatpak-builds app com.borgbase.vorta.yaml --force-clean && flatpak update com.borgbase.vorta -y
+$ flatpak run com.borgbase.vorta 
+```
+
+
+### macOS
+
+Clone the latest version of this repo:
 ```
 $ git clone https://github.com/borgbase/vorta/
 ```