Re-arrange CONTRIBUTING.md to add Flatpak description.

CONTRIBUTING.md

@@ -7,7 +7,27 @@ All contributions that improve Vorta for everyone are welcome. Before coding a n
 
 ## Local Development Setup
 
-### Linux
+Clone the latest version of this repo:
+```
+$ git clone https://github.com/borgbase/vorta/
+```
+
+Install in development/editable mode while in the repo root:
+```
+$ pip install -e .
+```
+
+Install additional developer packages (pytest, tox, pyinstaller):
+```
+pip install -r requirements.d/dev.txt
+```
+
+Then run as Python script. Any changes from your source folder should be reflected.
+```
+$ vorta
+```
+
+### Package with Flatpak
 
 Follow the setup guide on [flatpak.org](http://flatpak.org/setup/) to make sure you have ``flatpak`` and ``flathub`` installed.
 
@@ -22,12 +42,6 @@ Make a repo for your local builds (If you get the error: ``Remote listing for my
 ```
 $ 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):
@@ -46,28 +60,6 @@ $ flatpak run com.borgbase.vorta
 ```
 
 
-### macOS
-
-Clone the latest version of this repo:
-```
-$ git clone https://github.com/borgbase/vorta/
-```
-
-Install in development/editable mode while in the repo root:
-```
-$ pip install -e .
-```
-
-Install additional developer packages (pytest, tox, pyinstaller):
-```
-pip install -r requirements.d/dev.txt
-```
-
-Then run as Python script. Any changes from your source folder should be reflected.
-```
-$ vorta
-```
-
 ## Working on the GUI
 Qt Creator is used to edit views. Install from [their site](https://www.qt.io/download) or using Homebrew and then open the .ui files in `vorta/assets/UI` with Qt Creator:
 ```