Add comment to langversion usage

Signed-off-by: Erwin <erwinkramer@hotmail.com>

Author: erwinkramer

src/Directory.Build.props

@@ -26,6 +26,7 @@
     <SignAssembly>True</SignAssembly>
     <Deterministic>True</Deterministic>
     <TreatWarningsAsErrors>True</TreatWarningsAsErrors>
+    <!-- Keep C# 10 explicit while supporting netstandard2.0/netstandard2.1: older defaults break nullable and file-scoped namespaces. -->
     <LangVersion>10.0</LangVersion>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>    
     <EnablePackageValidation>true</EnablePackageValidation>