docs: add Steam Deck instructions to README

[kevin-wijnen]

Context

As mentioned previously, I've been working on trying to get Mesen working on my Steam Deck. Thanks to the effort being put in the CI pipeline, Mesen works very well with the AppImage nightly builds.

Albeit, there are some caveats related to Avalonia UI and Gamescope (with a possible solution being reverted) that warrant specific instructions how to use Mesen on the Steam Deck.

Hence why I created this PR to help other users with running Mesen on their Steam Deck.

Solution

I added the following to the README.md to aid Steam Deck/SteamOS users (it is hidden in normal view unless the user clicks on the arrow to unfold the SteamOS instructions):

• An outline of the issue with Avalonia UI and Gamescope related to the popups necessary to open up any menu of Mesen. Along with a workaround for those wanting to still use the menu in Game Mode.
• Installation instructions of the Linux - AppImage build.
• Instructions on how to do the first set up specifically on the Steam Deck (due to an issue with storing the config locally next to the Mesen AppImage).
• Recommended rebinds to access crucial features in Game Mode without any other workaround (Opening a file, pause and full screen).
• General troubleshooting of issues I came across myself (audio device not being automatically selected).
• Added instructions how to make game-specific shortcuts with Command-line Options.

[SourMesen]

Thanks for the PR!