Add comment clarifying why net* folders from Microsoft.NET.Build.Extensions package are excluded

Author: dsplaisted

build/MSBuildExtensions.targets

@@ -5,10 +5,20 @@
     <ItemGroup>
       <MSBuildExtensionsContent Include="$(GeneratedMSBuildExtensionsDirectory)/**/*" />
 
+      <!-- We want to include the tasks and targets from the Microsoft.NET.Build.Extensions package, but we don't want to include the DLLs
+           from that package under the net461, net462, etc folders.  That is because they come from the NETStandard.Library.NETFramework
+           package, and we want to insert them directly into the CLI from CoreFx instead of having to do a two-hop insertion (CoreFX -> SDK -> CLI)
+           if we need to update them.
+           
+           https://github.com/dotnet/sdk/issues/1324 has been filed to exclude these from the Microsoft.NET.Build.Extensions package when
+           we generate it.
+           -->
       <MSBuildExtensionsContent Include="$(MSBuildExtensionsNuPkgPath)\msbuildExtensions\**\*.*"
                                 Exclude="$(MSBuildExtensionsNuPkgPath)\msbuildExtensions\Microsoft.NET.Build.Extensions\net*\**" />
+
       <MSBuildExtensionsContent Include="$(MSBuildExtensionsNuPkgPath)\msbuildExtensions-ver\**\*.*"
                                 DeploymentSubpath="$(MSBuildExtensionsVersionSubfolder)/" />
+      
       <MSBuildExtensionsContent Include="$(NETStandardLibraryNETFrameworkNuPkgPath)\build\**\*.*"
                                 Exclude="$(NETStandardLibraryNETFrameworkNuPkgPath)\build\**\*.props;$(NETStandardLibraryNETFrameworkNuPkgPath)\build\**\*.targets"
                                 DeploymentSubpath="Microsoft.NET.Build.Extensions/" />