Add checklist to help find relevant error messages in a Proton log.

Checklist-Proton-bugs.md

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+# Checklist Proton bugs
+
+This is a checklist to help you find relevant error messages in your Proton log.
+
+## General guidance
+
+Search the Proton log for the following prefixes to find errors/warnings:
+  - ```fixme:```
+  - ```err:``` 
+  - ```warn:```
+  - ```error:```
+  
+---
+
+## Game won't start at all:
+
+### Steam2.dll
+Tracked at https://github.com/ValveSoftware/Proton/issues/2613 by @HonkingGoose.
+- ```warn:module:load_builtin_dll cannot open .so lib for builtin L"Steam2.dll":```
+
+### MFC42.dll
+Tracked at https://github.com/ValveSoftware/Proton/issues/2624 by @HonkingGoose.
+- ```err:module:import_dll Library MFC42.DLL```
+
+---
+
+## Missing cinematic/cutscenes:
+
+Problems with missing cinematics/cutscenes are most likely due to Media Foundation, Quartz or WMVcore issues. 
+These are tracked at https://github.com/ValveSoftware/Proton/issues/1464 by @HonkingGoose.
+
+### Media Foundation
+- ```mfplat.dll```
+- ```mfplay.dll```
+- ```mfreadwrite.dll```
+- ```mfstartup/dll```
+
+### Quartz
+- ```quartz.dll```
+- ```fixme:quartz:MPEGSplitter_QueryInterface```
+- ```err:quartz:GetClassMediaFile Media class not found```
+
+### WMVcore
+- ```WMVcore.dll```
+- ```fixme:wmvcore:WMReader_Open```
+- ```fixme:wmvcore:WMCreateSyncReader```
+
+### XNA
+
+- ```Microsoft.Xna.Framework.Video.dll```
+
+---
+
+## Audio playback issues:
+
+### Xaudio2
+Xaudio2 issues are labeled "XAudio2" by the moderator.
+The Proton log typically includes one of the following:
+
+- ```XAudio2_7.dll```
+- ```xaudio2_8.dll```
+
+---
+
+## Other errors:
+
+### .net
+.net issues are labeled appropriately by the moderator. 
+These include:
+- .NET-UE3 (Unreal Engine 3.)
+- .NET-winforms (System.Windows.forms toolkit)
+- .NET-WPF (PresentationFramework)
+- .NET-XNA (Uses XNA framework, implies Xaudio2 and possibly WMP)
+
+---