Nbgv could use better command line help

[the-ress]

I wanted to see what nbgv could do and it took me 4 attempts just to show the help:

Command	Result
nbgv	error: missing command
nbgv help	error: unknown command 'help'
nbgv /?	error: unknown command '/?'
nbgv --help	the actual help

1. nbgv should print the help when started without arguments. Or at least say direct the user in the error message.
2. The help should say something like: Run 'nbgv [command] --help' for more information on a command. (It isn't really obvious that the commands accept additional arguments.)
3. The help could include information about how to show it (--help, -h, -? and any others I missed)