update dotnet --help text

src/dotnet/commands/dotnet-help/HelpCommand.cs

@@ -35,8 +35,10 @@ Commands:
   pack          Creates a NuGet package
   migrate       Migrates a project.json based project to a msbuild based project
 
+Project modification commands:
+  add           Add items to the project
+
 Advanced Commands:
-  add           Group of commands - run `dotnet add --help` for more information
   nuget         Provides additional NuGet commands
   msbuild       msbuilds a project and all of its dependencies
   vstest        Runs tests from the specified files";