Update documentation to reflect more the current API

[Barbapapazes]

related to #46

Hello,

This issue will be used to track documentation update that we need to provide.

PR welcomes to help! (and do not hesitate to comment if you have more idea)

### Tasks
- [x] Sub command
- [x] Async command
- [x] Setup function
- [x] Cleanup function
- [x] Sub command can be a function
- [x] Sub command can be lazy loaded
- [x] Add the CLI to the bin

[NozomuIkuta]

@Barbapapazes

It would be worth adding "how to make the CLI available as 'binary'". I mean like, how to set up the CLI as bin?

// in development
node mycommand arg

// when published
mycli mycommand arg

[Barbapapazes]

@Barbapapazes

It would be worth adding "how to make the CLI available as 'binary'". I mean like, how to set up the CLI as bin?

// in development
node mycommand arg

// when published
mycli mycommand arg

Thanks for the feedback, that's a great idea!

[Barbapapazes]

@pi0, do you think we could add a section they use citty to the docs to highlight projects (like nuxt-ng) to help user to create their own?