Add Linux docs

LINUX.md

@@ -0,0 +1,25 @@
+## Linux Users
+
+No native Linux build exists for the game. Proton, however, works just fine. You can install it using the instructions below.
+
+Reminder that EAC or Referee builds will never be supported by Undead Universal Patch.
+
+### General process
+* Install Steam
+* Add Recroom_Release.exe as a non-steam game
+* Install `protontricks`; depends on your distribution (see sections)
+* Add the doorstop DLL using `protontricks --gui` [(more info)](https://linux-gaming.kwindu.eu/index.php?title=Installing_dlls)
+    - Select default wineprefix
+	- Install DLL
+	- Select the doorstop DLL `winhttp.dll`
+* Run the game through Steam
+
+### Arch Linux
+`protontricks` can be install from the [AUR](https://aur.archlinux.org/packages/protontricks)
+You should use the built-in Steam package [(instructions here)](https://wiki.archlinux.org/title/Steam) if you don't have Steam installed already.
+
+### Other distributions
+
+If you have any success doing this on Debian-based systems (Ubuntu 2x.x, elementary OS, etc)<br>
+or any other kind of system, add an issue and explain the process - I'll add it to this document.
+If you feel like making a pull request, that's okay as well.