Added DevelopmentDependency (#339)

Added <DevelopmentDependency>True</DevelopmentDependency> to build NuGet packages with the developmentDependency metadata

Author: paulomorgado

SourceLink.Copy.PdbFiles/SourceLink.Copy.PdbFiles.csproj

@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>netstandard1.4</TargetFramework>
     <IncludeBuildOutput>false</IncludeBuildOutput>
+    <DevelopmentDependency>True</DevelopmentDependency>
   </PropertyGroup>
   <ItemGroup Label="dotnet pack instructions">
     <PackageReference Update="NETStandard.Library" PrivateAssets="all" />
@@ -19,4 +20,4 @@
       <PackagePath>build</PackagePath>
     </Content>
   </ItemGroup>
-</Project>
+</Project>