Created Roslyn Analyzers (third-party) (markdown)

Roslyn-Analyzers-(third-party).md

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+As a reminder, the project files' include graph looks like:
+```
+/Common.props
+├─ /ExternalProjects/LibCommon.props
+│   ├─ /ExternalProjects/FlatBuffers.GenOutput/FlatBuffers.GenOutput.csproj
+│   ├─ /ExternalProjects/iso-parser/ISOParser.csproj
+│   ├─ /ExternalProjects/NLua/NLua.csproj
+│   ├─ /ExternalProjects/PcxFileTypePlugin.HawkQuantizer/PcxFileTypePlugin.HawkQuantizer.csproj
+│   └─ /ExternalProjects/AnalyzersCommon.props
+│       ├─ /ExternalProjects/BizHawk.Analyzer/BizHawk.Analyzer.csproj
+│       └─ /ExternalProjects/BizHawk.SrcGen.ReflectionCache/BizHawk.SrcGen.ReflectionCache.csproj
+├─ /src/BizHawk.Version/BizHawk.Version.csproj
+├─ /src/MainSlnCommon.props
+│   ├─ /src/BizHawk.Common/BizHawk.Common.csproj
+│   ├─ ... (other main solution projects, including tests)
+│   └─ /src/MainSlnExecutable.props
+│       ├─ /src/BizHawk.Bizware.Test/BizHawk.Bizware.Test.csproj
+│       ├─ /src/BizHawk.Client.DiscoHawk/BizHawk.Client.DiscoHawk.csproj
+│       └─ /src/BizHawk.Client.EmuHawk/BizHawk.Client.EmuHawk.csproj
+└─ /ExternalToolProjects/ExtToolCommon.props
+    ├─ /ExternalToolProjects/HelloWorld/HelloWorld.csproj (also imports NET48ExternalToolForm.targets)
+    └─ ... (other ext. tools)
+```
+`/Common.props` enables the .NET Analyzers and StyleCop, and imports BizHawk.Analyzer and some third-party Analyzers.
+
+You can tell the assembly references are working if `;;` is flagged with "MA0037: Remove empty statement".
+You can tell the ruleset reference is working if the above is true but the output isn't flooded with other warnings like "MEN002: Line must be no longer than 160 characters".
+Similarly, you can tell the StyleCop config reference is working if the output isn't flooded with other warnings like "SA1600: Elements should be documented".
+
+> Note: `/src/MainSlnCommon.props` *disables* Analyzers unless `-p:RunAnalyzersDuringBuild=true` is passed to `dotnet` / the build script.