Add note about different commands within and outside of app

[cllns]

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:

% 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.