netfx targeting pack

Author: mgravell

Directory.Build.props

@@ -49,6 +49,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="all" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All"/>
     <PackageReference Include="Nerdbank.GitVersioning" Version="3.1.91" PrivateAssets="all" />
   </ItemGroup>