dotnet --help should be more helpful when trying to run a published app

[Petermarcu]

This is tracking this comment here: dotnet/core#77 (comment)

[livarcocc]

We have added documentation in the CLI docs for this. You can see @mairaw's PR if you follow her comments from the link above.

However, I think we should also add the info that you can do dotnet <your_published_app_dll> to the output of dotnet --help itself.

cc @richlander

[livarcocc]

This information has now been added to dotnet --help.