README.md: add RetroArch to other apps section

[Source](https://github.com/libretro/RetroArch/pull/13339)

README.md

@@ -73,6 +73,7 @@ Other apps which can integrate with GameMode include:
 * [ATLauncher](https://atlauncher.com/downloads) Minecraft launcher
 * GNOME Shell ([via extension](https://github.com/gicmo/gamemode-extension)) - indicates when GameMode is active in the top panel.
 * Lutris - Enables GameMode for all games by default if available (must have both 32- and 64-bit GameMode libraries installed), configurable in preferences.
+* [RetroArch](https://www.retroarch.com) - is a frontend for emulators, game engines and media players.
 
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 ## Development [![Build and test](https://github.com/FeralInteractive/gamemode/actions/workflows/build-and-test.yml/badge.svg)](https://github.com/FeralInteractive/gamemode/actions/workflows/build-and-test.yml)