Add note about different commands within and outside of app

We only allow `hanami new` outside of existing Hanami projects, and only allow almost all the other commands inside a Hanami 2 project (`version` is rightfully allowed in both).

For example:
```shell
% hanami version
v2.0.0.beta3
% hanami help
Commands:
hanami new APP
hanami version
% hanami new beta_three
% cd beta_three
% hanami help
Commands:
hanami console # App REPL
hanami generate [SUBCOMMAND]
hanami install
hanami middlewares # List all the registered middlewares
hanami routes # Inspect application
hanami server
hanami version
```

Something like: "There are more commands available when you're inside of an app, but we've detected that you're not in a Hanami 2 folder". We could even list them (on one line, without descriptions) too. And when inside an app something like "`hanami new` is also available when you're outside of an existing Hanami project folder."

I think this could be a "good first issue" for newcomers who'd like to contribute to the Hanami 2 effort.