Add notification of potential difference in sample csproj

docs/guides/getting_started/samples/project.csproj

@@ -1,5 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
+  // The following may differ depending on the latest version of 
+  // .NET Core Framework or Discord.NET.
+  
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <TargetFramework>netcoreapp1.1</TargetFramework>