Readme: add Steam Deck instructions

README.md

@@ -144,6 +144,13 @@ flatpak install flathub io.github.NhekoReborn.Nheko
 guix install nheko
 ```
 
+#### Steam Deck
+
+Install nheko via the `Discover` app in Desktop Mode (this installs the flatpak). To also make it work in Game Mode you'll have to add it as a "Non-Steam Game" to steam and than add the following `--env` variables to the `flatpak run` command. This is necessary for nheko to connect to KWallet correctly. The resulting arguments field in the steam settings should look something like this.
+```
+run --env=XDG_CURRENT_DESKTOP=KDE --env=KDE_SESSION_VERSION=5 --branch=stable --arch=x86_64 --command=io.github.NhekoReborn.Nheko --file-forwarding io.github.NhekoReborn.Nheko @@u @@
+```
+
 #### macOS (10.14 and above)
 
 with [homebrew](https://brew.sh/):