Add comment for nupkg folder holder file.

osu.Framework.NativeLibs/osu.Framework.NativeLibs.csproj

@@ -2,14 +2,13 @@
   <PropertyGroup Label="Project">
     <TargetFramework>netcoreapp3.0</TargetFramework>
     <OutputType>Library</OutputType>
-    <ContentTargetFolders>contentFiles</ContentTargetFolders>
     <AssemblyTitle>osu!framework Libraries</AssemblyTitle>
     <AssemblyName>osu.Framework.NativeLibs</AssemblyName>
-    <Description>Native libraries for osu!framework</Description>
   </PropertyGroup>
   <PropertyGroup Label="Nuget">
     <IsPackable>true</IsPackable>
     <Title>osu!framework Libraries</Title>
+    <Description>Native libraries for osu!framework</Description>
     <PackageId>ppy.osu.Framework.NativeLibs</PackageId>
     <PackageTags>osu game framework libraries</PackageTags>
     <IncludeBuildOutput>false</IncludeBuildOutput>
@@ -20,6 +19,10 @@
       <PackagePath>runtimes</PackagePath>
     </None>
     <None Include="_._">
+      <!-- Means this package doesn't provide any reference assembly to the target framework.
+           nupkg is a zip file and doesn't has concept for folders,
+           so there must be something under the path, otherwise client will consider this package broken.
+           See https://docs.microsoft.com/en-us/nuget/reference/errors-and-warnings/nu5128#scenario-2 . -->
       <Pack>true</Pack>
       <PackagePath>lib\$(TargetFramework)</PackagePath>
     </None>