Mention how to manually start xdg-desktop-portal in FAQ

Signed-off-by: falkTX <falktx@falktx.com>

README.md

@@ -30,7 +30,6 @@ Though currently the following should be noted:
 - Keyboard input does not always work in some hosts [#24](https://github.com/DISTRHO/Cardinal/issues/24)
 - VST3 support incomplete/experimental [#41](https://github.com/DISTRHO/Cardinal/issues/41)
 - Windows 32bit builds do not work well [#80](https://github.com/DISTRHO/Cardinal/issues/80)
-- Windows High-DPI issues [#186](https://github.com/DISTRHO/Cardinal/issues/186)
 
 ### Stable release
 

docs/FAQ.md

@@ -64,9 +64,15 @@ As a plugin, the state will be saved together with the host/DAW project.
 ## On BSD/Linux/X11 the menu item "Save As/Export..." does nothing
 
 The save-file dialogs in Cardinal requires a working [xdg-desktop-portal](https://github.com/flatpak/xdg-desktop-portal) DBus implementation from your desktop environment.  
-Typically your desktop already provides this, if not consider looking for a package to install with "desktop-portal" in the name.
+Typically your desktop already provides this, if not consider looking for a package to install with "desktop-portal" in the name.  
+If you are running a window manager without a "real" desktop environment (like custom X11 or i3 setups),
+you will need to manually activate the systemd unit that provides these DBus services, like so:
 
-The open-file dialogs in Cardinal do not have this restriction, with a fallback in case desktop portal is not available.
+```
+systemctl enable xdg-desktop-portal --user --now
+```
+
+Note: The open-file dialogs in Cardinal do not have this restriction, with a fallback in case the desktop portal is not available.
 
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