Started to document the build tool.

next steps:
- Lookup `BuildConfiguration` and see if it should have a breakdown
- For the "Parse and build targets" prioritize local builds or now and ignore the remote builds sections since its only for mac.
- Figure out what `DirectoryReference` is and what `QueueProjectDirectory` does from `FileMetadataPrefetch`.
- Quickly skim `SourceFileWorkingSet` and breakdown the `Build` function of `BuildMode`

docs/Build & Automation System/UBT.md

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+# Unreal Build Tool
+
+Located in `Engine\Source\Programs\UnrealBuildTool\`  
+C# solution.
+
+Main Loop:
+
+Located in `UnrealBuildTool.cs`
+
+1. Start peformance info capture
+2. Parse command line arguments
+3. Parse global options
+4. Logging & UBT assembly setup
+5. Set working directory to Engine/Source
+6. Setup build mode
+7. Setup tool mode options
+8. Get engine directory contents
+9. Read XML Configurataion
+10. Create UBT run file
+11. "Lock Branch"
+12. Register build platforms
+13. Create ToolMode which will handle the rest
+14. Close out
+
+## Global Options
+
+Also located in same file.
+
+Offical inline documentation:  
+> Global options for UBT (any modes)
+
+Commandline options are deffined using Epic's `CommandLineAttribute`.
+
+## ToolMode
+
+Offical inline documentation:  
+> Base class for standalone UBT modes. Different modes can be invoked using the -Mode=[Name] argument on the command line, where [Name] is determined by the ToolModeAttribute on a ToolMode derived class. The log system will be initialized before calling the mode, but little else.
+
+-Mode is defined in GlobalOptions definition. 
+
+Has a single function:
+
+```csharp
+public abstract int Execute(CommandLineArguments Arguments, ILogger Logger);
+```
+
+## BuildMode
+
+Derived from ToolMode, used to build a *target*.
+
+`Execute` procedure Flow:
+
+1. Output arguments & setup the logger
+2. Read the xml configuration files
+3. Apply architecture configs (platform specific configs for a target)
+4. More logging setup
+5. Create build configuraiton object
+6. Parse and build targets
+    - Pase all target descriptors
+    - Clean all targets with `CleanMode` (a tool mode) that have `bRebuild` flagged.
+    - Handle remote builds (Seems to be excluisvely Mac)
+    - Handle local builds
+       - Get all project directories & build options
+       - For each project: create a `SourceFileWorkingSet` object and `Build`.
+
+

docs/Launch/mainloop.md

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+# Main Engine Loop
+

docs/journey.md

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 # Journey
 
+Starting with the build system and main engine loop.
+
+[Build System](Build%20&%20Automation%20System/UBT.md)
+
+[Main Engine Loop](Launch/mainloop.md)