Document enableamdags/PROTON_ENABLE_AMD_AGS as runtime compat config option

[eXhumer]

enableamdags as compat config string / PROTON_ENABLE_AMD_AGS as environment variable was added with commit 7da8928, which allows a lot of games with HDR support to work correctly on Steam Deck OLED, however this option is not documented in the README.

I own the game on Ubisoft Launcher and was looking Assassin's Creed Odyssey HDR support on Steam Deck OLED when I came across issue #7292 suggesting I can add this option to fix the HDR and it actually worked. I would appreciate if this option gets documented to README, thanks.