'dotnet add' with no arguments should probably show output of `dotnet add --help`

[DustinCampbell]

This would be consistent with the behavior of dotnet add package and dotnet add reference when called with no other args.

Steps to reproduce

dotnet add

Expected behavior

Required command was not provided.

.NET Add Command

Usage: dotnet add [arguments] [options] [command]

Arguments:
  <PROJECT>  The project file to operate on. If a file is not specified, the command will search the current directory for one.

Options:
  -h|--help  Show help information

Commands:
  package    Command to add package reference
  reference  Command to add project to project reference

Use "dotnet add [command] --help" for more information about a command.

Actual behavior

Required command was not provided.

Environment data

dotnet --info output:

.NET Command Line Tools (1.0.0-rc4-004530)

Product Information:
Version: 1.0.0-rc4-004530
Commit SHA-1 hash: 4f0b7bd

Runtime Environment:
OS Name: Windows
OS Version: 10.0.15007
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\1.0.0-rc4-004530