Create documentation

[Esemesek]

Create cli documentation. Official react-native docs don't provider enough information about all commands, functionalities and examples.

[Kielan]

We need a website, i hope I can be the one to find time to make the react-native-cli docs ^^ that would be cool, or someone else.

[cito125]

I agree, docs would be super useful.

[morenoh149]

Are you open to using GitHub pages? At least add installation instruction in the readme. How do you install this?