warn if package is not release configuration

Author: smdn

src/Directory.Build.targets

@@ -39,4 +39,8 @@ SPDX-License-Identifier: MIT
       <AssemblyMetadata Include="RepositoryCommit" Value="$(RepositoryCommit)" Condition=" '$(RepositoryCommit)' != '' " />
     </ItemGroup>
   </Target>
+
+  <Target Name="WarnNupkgConfiguration" AfterTargets="Pack" Condition=" '$(Configuration)' != 'Release' ">
+    <Warning Text="generated nupkg with configuration '$(Configuration)'" />
+  </Target>
 </Project>