Merge pull request UltraStar-Deluxe#802 from Bronkoknorb/patch-1

Add information about SongDir configuration for flatpak

README.md

@@ -34,6 +34,7 @@ Currently, the following installation channels are offered:
 - The primary folder for songs on OS X is `$HOME/Music/UltraStar Deluxe`, which is created when UltraStar Deluxe is run for the first time.
 - On OS X, by default the `config.ini` file is created in `$HOME/Library/Application Support/UltraStarDeluxe` when UltraStar Deluxe is run for the first time.
 - When running in borderless fullscreen mode, the monitor it runs on can be configured by setting `Graphics.PositionX/Y` to an offset in pixels.
+- If installed via the flatpak package, the primary song folder is `~/.var/app/eu.usdx.UltraStarDeluxe/.ultrastardx/songs/` and the config.ini is located in `~/.var/app/eu.usdx.UltraStarDeluxe/.ultrastardx/` by default. To configure additional song directories, they first need to be made accessible to the flatpak app using the command: `flatpak override eu.usdx.UltraStarDeluxe --filesystem=/your/new/songfolder` - Afterwards, the directory can be added to the config.ini file as usual.
 
 ### 4. Further documentation
 The [wiki](https://github.com/UltraStar-Deluxe/USDX/wiki) contains more information on: