Add Flatpak usage notes to readme

README.md

@@ -57,11 +57,16 @@ sudo dnf install mangohud
 
 #### Flatpak
 
-If you are using Flatpaks, you will have to add the [FlatHub repository](https://flatpak.org/setup/) for your specific distribution, and then, to install it, execute:
+If you are using Flatpaks, you will have to add the [Flathub repository](https://flatpak.org/setup/) for your specific distribution, and then, to install it, execute:
 
+##### For Steam flatpak
 ```
 flatpak install com.valvesoftware.Steam.VulkanLayer.MangoHud
 ```
+To enable MangoHud for all Steam games:
+```
+flatpak override --user --env=MANGOHUD=1 com.valvesoftware.Steam
+```
 
 # Normal usage